Culture and the Organization of Infant Sleep: A Study in the Netherlands and the U.S.A.

Abstract

This study investigates differences in the amount and structure of infant sleep in two cultural places with previously documented, divergent parental beliefs and practices. Eight-month-old infants (n=24 per site) were recruited from towns in the Netherlands and the eastern U.S.A. To evaluate sleep, infants’ physical activity was recorded at home for 24 hours using a miniature actigraph, while parents kept a diary of infant activities. Measures derived from actigraphy include total sleep, longest sleep episode, longest wake episode, number of sleep episodes, and percent of sleep during nighttime, as well as time in the stages of Quiet and Active Sleep. Measures based on the parental diaries include most of these aspects as well, except those related to sleep stages. Results based on the more precise actigraphy method indicate that (1) the Dutch infants averaged 13.65 hours of sleep per 24 hours, 1.67 hours more than the U.S. infants; this difference was mostly due to daytime sleep; (2) The Dutch infants’ longest wake episode averaged less than that of the U.S. infants, while their longest sleep episode appeared slightly longer. (3) The Dutch infants, compared to the U.S. sample, spent more time in the Quiet, rather than the Active phase of sleep; (4) They began their Quiet sleep earlier in the evening than did their U.S. counterparts. Measures derived from parental diaries are largely in agreement with the actigraph findings. These results are consistent with reported and observed practices and beliefs in the two communities. The pattern of differences – less apparent maturity among the Dutch in the amount of sleep, but greater apparent maturity in the structure of sleep -- illustrates that behavioral and neurological maturity can be assessed only in the context of the developing child’s adaptation to the specific demands and affordances of the culturally structured developmental niche.

1. Introduction

The development of sleep in infancy is a fundamental maturational process (Anders, Sadeh, & Appareddy, 1995; Henderson, Grance, Owens, & Blampied, 2010). Its importance is highlighted by studies of the correlates and consequences of insufficient sleep, for both children (El-Sheikh, Kelly, Buckhalt, & Hinnant, 2010) and adults (Liu & Chen, 2019). Unlike the case for more mature individuals, however, there is virtually no experimental research on sleep adequacy in infants (one exception: Canet, Gaultier, D’Allest, et al., 1989). In correlational and quasi-experimental studies, outcome measures early in life are difficult to establish, and it is a challenge to disentangle causal factors. When parents ask, “How much should my baby be sleeping?”, pediatricians, nurses, psychologists, and experts in sleep health generally refer to normative data. One popular website, for example, answers “How much [sleep do babies] actually need for their health and development?” with assurances that “By about 4 months, most babies sleep 10 to 12 hours at night” and at 6 to 12 months “babies typically sleep about 11 hours at night.” (O’Connor, 2020). With maturity, parents are assured, sleep will consolidate in the night hours, and eventually even naps will cease.

The scientific inadequacy of using norms as universal guidelines for sleep is evident from the cross-cultural literature. These reports indicate, for example, that infants and toddlers in New Zealand typically sleep 13.31 hours per 24, compared to 11.62 in Japan (Mindell, Sadeh, Wiegand, et al., 2010). It may be helpful from a practical point of view to know what is typical in a particular locale, but given the apparent global variability, such numbers – if accurate -- do not suggest a true, meaningful, pan-human norm. In addition, there are significant difficulties in both establishing and interpreting cross-cultural comparisons. On the methodological side, the frequently cited, large-scale, multi-national studies (Hense et al., 2011; Mindell et al., 2010) rely on parental report (e.g. questionnaires), a method of uncertain validity (Sadeh, Acebo, Seifer, et al., 1995; Werner, Molinari, Guyer, et al., 2008). Such studies may speculate on the causes of different sleep patterns, or pursue explanations based on correlations with national-level information, but they generally lack direct measures of potential influences on the infants’ daily lives.

The present report takes a different approach by focusing on aspects of infants’ culturally structured environments, or developmental niches, as their immediate contexts of development. As we have noted elsewhere (Super & Harkness, 1986), the developmental niche is a theoretical framework constructed for the purpose of conceptualizing the distinct components of the context of children’s daily life, the inter-relationships of the components with each other, the role of the child as an active agent in his or her own development, and the influence of the wider socio-cultural milieu within which children and families are located. Three distinct components, or sub-systems, constitute the developmental niche: 1) The physical and social settings of the child’s daily life; 2) Parents’ (and other caregivers’) customs and practices of care; and 3) The psychology of the caretakers, including culturally shared beliefs about child development, the family, and themselves as parents – or what we have termed “parental ethnotheories” (Harkness & Super, 1996). The child’s development within the niche is influenced bi-directionally, as aspects of the child (e.g. age, gender; personal qualities) affect the ways that parents and other caretakers respond to the child, and construct the child’s immediate environment. Influences of the wider socio-cultural environment, such as family policies and parents’ work conditions, may influence particular components of the niche (e.g. settings of daily life), which then interact with the other two components. We and others have found that the developmental niche framework provides a useful heuristic device for a detailed, up-close examination of the role of children’s environments as they influence development. Study of the child’s developmental niche may be particularly relevant in relation to universal aspects of development such as motor skills (Super, 1976), first language acquisition (Harkness, 1977), and sleep (Super & Harkness, 1982).

Ethnographic background

We first became aware of differences between the developmental niches of Dutch and U.S. infants in field work in 1992, during a comparative study of parental ethnotheories and children’s development in the opening years of life ( Harkness et al., 2000; Super et al., 1996). Diaries that parents kept for a week, documenting their child’s time in various settings with their accompanying activities and cast of characters, indicated that at the age of 6 months, the Dutch babies in our study were sleeping two hours more per 24-hour day than were the U.S. babies (a difference that diminished with increasing age of the children in these cross-sectional samples, to merely 20 minutes for the four-year-old children: Super et al., 1996, p. 457). While awake, systematic observations showed that the Dutch babies were more frequently in a state of Quiet Arousal than in a state of Active Arousal, while the opposite was true of a comparison sample of U.S. babies (Super et al., 1996, p. 458).

Customs and practices of care for infants in these two cultural samples also showed contrasting patterns, including earlier and more regular bedtimes for the Dutch infants (Super et al., 1996, p. 457). Observations and interviews of the Dutch mothers indicated that they were generally providing lower levels of stimulation than were the U.S. mothers (Super et al., 1996, p. 458; Harkness & Super,1997). For example, one mother described a morning routine of giving her 6-month-old a bottle and then putting her in an infant seat nested within a playpen for about half an hour so she wouldn’t move around a lot and spit up, after which she would lay the baby in the playpen itself to play with a few simple toys such as teething rings, until time for her morning nap.

These cultural patterns in settings and practices of care for the Dutch infants were evidently motivated by strong culturally shared beliefs, as we soon discovered through interviews with our sample of Dutch parents. We first encountered the “three R’s” – rust (rest), regelmaat (regularity), and reinheid (cleanliness) – in the 1992 field work, and returned to study the topic in more detail in 1995–96 (Harkness et al., 2007; van Schaik et al., 2020). The “three R’s” are a well-articulated set of guiding principles that emphasize the importance of sleep and regularity during infancy and childhood, and indeed across the lifespan. The underlying philosophy is deeply rooted in Dutch culture. As early as the 16^th century: “[t]he neat cleanliness of the province of Holland was notorious among travelers … In 1567 the Florentine Lucovico Guiccardini noticed ‘order and tidiness everywhere’” (Pye, 2015, p. 176). Sleep was a particular focus in the writings of the 17^th century Dutch physician Johann van Beverwijck (1649). The “three R’s” were codified as such by the end of the 19^th century (van Hulst, 1905), and by 1907 some 15,000 copies of a parenting guide with that title had been distributed in the Netherlands (Rose, 1907). Related practices have been noted by visitors at various times in the past: “The Dutch keep their children in a state of repose” observed an American domestic expert in 1831 (Child, 1831/1992, p. 1); over a century later, the developmental psychologist Freda Rebelsky (Rebelsky & Dorman, 1973, p. 384) reported that Dutch mothers believed “that a baby [of 3 months] should sleep and not play or stay awake.” The “three R’s” are still widely espoused by parents and professionals, as both rest and regularity are often promoted by the national well-baby clinics, and in Dutch pediatric research (Blom, van Sleuwen, de Vries, 2009). Detailed analyses of mothers’ beliefs about early sleep reveal Dutch mothers’ emphasis on regularity in the baby’s daily life, and on common practices to establish schedules for sleeping, eating, and time outside (Harkness et al., 2007; van Schaik et al., 2020).

In contrast, the importance of “stimulation” – and particularly stimulation for “early cultivation of the intellect” (Child, 1831/1992, p. 10) -- has deep historical roots in the United States, and was more recently formalized in the 1960s cognitive revolution in psychology. “Stimulation” resonates with American cultural themes, including the active individual who engages and masters the natural environment (Kluckhohn & Strodtbeck, 1961). In an interesting example of mutual influence between popular and professional culture in the U.S., the various meanings of “stimulation” began to come together in broad synergy during the 1960s, as stimulation came to be a response to “cultural deprivation” (Aaron, 1962), an element in pursuing “excellence” (Gardner, 1984), and a technique to “boost your baby’s brain power” (Engle-Smothers & Heim, 2009). A detailed analysis of maternal interviews demonstrates significantly greater attention by U.S. mothers to the importance and practice of stimulation, especially to advance cognitive development, in comparison to mothers from the Netherlands and several other cultural sites in Europe and Asia (Harkness et al., 2007).

In addition to these widely recognized contrasting cultural themes, other mostly implicit ethnotheories play a role in directing parenting practices. In the present context, as a detailed analysis of mothers’ talk about rest and regularity for U.S. and Dutch mothers of six-month-old infants shows, these two groups of mothers held differing assumptions about the extent to which infants can be trained to keep regular sleep schedules, as opposed to seeing this aspect of development as largely determined by the infant. As van Schaik and her colleagues state, the Dutch mothers “seemed comfortable with the idea that even young babies can be expected to adjust their own schedule to external demands, such as the anticipated schedule of daycare and the needs of other family members including themselves (van Schaik et al., 2020, p. 21).” In contrast, “the U.S. mothers generally saw themselves as playing a much less active role in guiding their baby toward a regular routine. Although they often indicated they hoped their infant would develop a routine sooner rather than later, particularly with regard to sleep, they were less likely to feel that they could - or even should - enact strategies to establish a regular schedule in the early months. Keenly observing the baby’s patterns as they emerged, these mothers tended to construct their own days accordingly” (p. 22).

Beyond the direct effects of the infant’s developmental niche, other features of the wider environment play a role in shaping the niche itself. In the present case, for example, Dutch national policies supported an adequate family income based on only one breadwinner (generally the father), and local expectations of employers to more accommodate part-time employment. These differences are evident in the demographic parameters of mothers in the present study, but they do not directly affect the results, as described below.

1.2. The Present Study

In light of our earlier findings and the available historical material, the present study focuses on two questions. First and most simply, would the patterns of sleep duration and timing found earlier correspond to new analyses including the more accurate measures using an actigraph? Our earlier study of sleep (Super et al., 1996) was based solely on mothers’ diaries, which have been shown in at least some samples to yield accurate estimates for some sleep measures, but not for total sleep, presumably because parents are not aware of some night wakings and thus overestimate sleep time (Werner et al.., 2008). There is no way to ascertain from the 1996 data whether or not over-estimation is of the same magnitude in the two groups. In contrast, the findings presented here are based on a more precise and objective methodology, namely actigraphy (Sadeh et al.., 1995; Werner et al.., 2008).

More profoundly, the second question we wished to investigate in the present study is whether the observed differences in sleep behavior have their counterpart in the inner “architecture” or structure of sleep, which can be reliably distinguished in actigraph recordings (Sadeh et al.., 1995). The two recognized phases -- Quiet Sleep (QS) and Active Sleep (AS) – are distinct behaviorally, hormonally, neurologically, and physiologically (Blanco-Centurion, 2019; Frasch, Zwiener, Hoyer, et al., 2007; Mrowka, Cimponeriu, Patzak, et al., 2003). QS is characterized behaviorally by regular breathing, eyes closed without rapid eye movements, and very little body movement, while EEG recordings show spindles and slow wave activity. QS increases compared to Active Sleep (AS) during the first two years of life (Emde, 2006). This shift is thought to reflect both maturation of the necessary, uniquely complex inhibitory and vertical feedback mechanisms, as well as a decline in the need for sustained brain activity during sleep, as increasing quiet-alert periods while awake provide increasing sensory input (Dennenberg & Thoman, 1981; Roffwarg, Muzio, & Dement, 1996). For these reasons, QS has been considered an especially useful index of brain maturation (Beckwith & Parmelee, 1986; Cailleau et al., 2020; Dereymaeker, 2017; Feldman & Eidelman, 2003; Parmelee & Stern, 1972). How the proportional development of QS and AS might fare under the two contrasting cultural settings studied here is a novel question. In the absence of information other than the overall more restful picture of wake and sleep in our earlier Dutch samples, we expected that the Dutch infants would have a greater amount of QS than their U. S. peers.

2. METHOD

2.1. Participants and settings

This report is part of the International Baby Study (IBS), focusing particularly on Dutch and U.S. families. Our intent in recruitment was to include members of two separate, recognizable cultural communities: middle-class New England Americans, and middle-class, ethnic Dutch residing in towns of the central provinces of the Netherlands. Because the logic of this comparison centers on locally shared cultural models and community practices, without atypical family disruptions or stresses, the samples were restricted to families in which (1) parents were native-born and native speakers of the local language, (2) one or both parents were employed, and (3) there were no serious health problems.

The U.S. sample consists of 24 families recruited through obstetric and pediatric services, as well as social networks of initial participants, in Eastern Connecticut and south-central Massachusetts. These families are similar in most ways to our earlier sample from metropolitan Boston (Super et al., 1996), although they generally lived in smaller, less densely populated towns. They owned or rented their own, one- or two-family homes, in most cases surrounded by small yards of one-eighth acre or less. Although this housing pattern provided immediate access to the outdoors and open play space, the U.S. families were in some other regards rather isolated. They generally had to drive for shopping, delivering older children to school, or visiting friends or family. Some of the mothers had relatives in the same or a nearby town, but rarely in the same neighborhood. They were nearly all employed at least part-time before birth of the child enrolled in this study, and most had resumed work by the baby’s second month, as paid maternity leave was limited. They were employed in a broad range of jobs, including social worker, attorney, foster parent, sales clerk, and teacher. The fathers, too, worked in varied occupations, including truck driver, professor, attorney, software technician, and business manager. None of the infants in the present study was enrolled in center-based daycare, but nonmaternal care in the home was common, most frequently by a hired babysitter. All the babies had their own bedrooms, where they were usually put to sleep both for the night and for daytime naps. On occasion, the baby napped in the mother’s car as she drove out for errands; or sometimes specifically to induce sleep (see Table 1).

Table 1.

Sample Characteristics

Measure	NL	US	Difference	Test Statistic	p
Total n	24	24
Infant sex (percent male)	53%	69%	16%	X2=1.02	.31
Infant birth order (percent first)	52%	52%	0%	X2=0.00	ns
Infant age at observation (median)	8.33	8.43	0.10	z = 0.32	.75
Percent recordings on weekend	17%	9%	8%	X2=.77	.38
Father’s Hollingshead Occupation Score (median)	3.0	3.0	0	z = 0.00	ns
Mother’s age (years)	28.7	33.4	4.7	t=−3.05	.004
Mother’s education (years)	15.0	17.3	2.3	t=−3.35	.002
Mother’s employment
Full time (>=35 h/week)	9%	52%		X2=11.03	.004
Part-time (< 35 h / week)	50%	14%
Not employed	41%	33%
Hours of non-maternal care (7 am - 7pm) (median)	0.98	2.97	1.99	z = 1.70	.09
Percent non-maternal care by:
employee	21%	43%
father	43%	30%		X2 = 4.24	.12
other relative	36%	26%
Usual location of initial putting to sleep at night
Baby’s room	81%	89%
Parents’ room	5%	5%		Fisher’s Exact	.54
Other room (e.g. living room)	14%	6%
Usual location of daytime naps
Baby’s room	66%	73%
Parents’ room	5%	0%
Other room (e.g. living room)	13%	19%		Fisher’s Exact	.09
Outside	16%	0%
In car	0%	8%

In the Netherlands, study families resided in towns surrounding the city of Utrecht, about 25 miles from “Bloemenheim,” our earlier research site and part of the same cultural area. The Dutch families were recruited through personal networks and professional contacts of Masters-level students at the Utrecht University. As with our earlier Dutch samples (Harkness et al., 2007; Super et al., 1996; van Schaik et al., 2020), most families lived in two- or three-level townhouses or in apartments, often facing out onto a small park or other green space. Shops and schools were within easy walking or biking distance, and most families had friends and relatives nearby. Slightly more than half of the Dutch mothers were employed at the time of this research, in occupations such as teacher, nurse, or sales clerk. The fathers’ occupations included teacher, manager, and gardener. As with the U.S. sample, no infants attended a daycare facility; non-maternal care at home was most frequently provided by the father or other relative. As in the U.S., all the babies had their own bedroom, and were generally put there to sleep, both at night and during the day. Sometimes, however, they were set outside in a baby carriage for a nap, well covered, in the belief that fresh air – cool or even cold – was healthy for children (see Table 1).

The samples were located in particular communities within their respective countries. For convenience we refer to these samples by their nationalities, but the focus on particular places should be kept in mind when interpreting the results. They are not necessarily representative of their country’s population, just as samples in most monocultural studies are not demographically accurate representations of the entire nation from which the sample is drawn. The question of how generalizable our results might be to other regions or populations is beyond the scope of the present research, although some insight can be gained from noting the similarity of the present results to other reports (Harkness et al., 2007; Huitron ´ et al., 2019; Rebelsky & Dorman, 1973; Super et al., 1996; van Schaik et al., 2020).What we highlight here is differences in objectively measured infant sleep patterns in the two cultural samples.

As indicated in Table 1, the two cultural samples are the same or very similar with regard to the infants and their patterns of care. Sex, birth order, age at assessment, and fathers’ average Hollingshead Occupation scores (Hollingshead, 1975) are all essentially the same, as is the percentage of recordings made on weekends. Sleeping arrangements were generally similar for the two samples: the vast majority of babies were put to bed at night in their own rooms. Daytime naps were also predominantly in the baby’s room, although napping outdoors was another choice uniquely for the Dutch parents, while U.S. infants occasionally napped in the car, unlike the Dutch infants. In contrast, mothers in the Dutch sample averaged nearly five years younger than the U.S. mothers, had completed about two years less formal schooling, and were less likely to be employed full-time. Correspondingly, average hours of non-maternal care for the U.S. infants were three times those of the Dutch infants, and fathers and other relatives were more commonly (although not significantly) caretakers in the Dutch sample. These education and employment differences reflect a more general contrast between the two societies, but as will be shown, their inclusion in our analyses does not alter the conclusions.

2.2. Procedures and data calculation

2.2.1. Actigraphy

Infants’ physical activity was recorded in the home over at least a 24-hour period using a miniature actigraph attached to the baby’s ankle, while parents kept a diary of the infant’s activities. Parents were asked to select a day for the recording shortly after their infant’s 8-month birthday that did not include any unusual events, and was not a holiday. A project researcher visited the home in advance to review the nature of the work, to obtain signed consent (approved by the two relevant university Institutional Review Boards), and to explain use of the diary form and of the actigraph.

The actigraph (Ambulatory Monitoring, 1998) is a small device sensitive to movement (somewhat like a pedometer), and it stores a digital representation of activity level summed over each minute of use. Recordings of the infant’s physical movement were uploaded to a computer and then reduced to minute-by-minute scoring of sleep and wakefulness using the algorithm established by Sadeh and colleagues (Sadeh et al., 1995). Before further analysis, the scored record was reviewed and hand-corrected for any instances where diary information indicated a clear recording artifact, such as the actigraph having been removed temporarily for the infant’s bath.

In establishing the validity of the algorithm for use with infants, Sadeh and colleagues compared actigraph results to continuous observations by trained observers. The computerized scoring agreed with the observers 83% of the time across the three sleep stages used here (awake, QS, and AS). Reliable measures of individual differences in sleep may require up to seven consecutive nights of recording (Acebo et al., 1999), but the assessment of a group or group differences does not require the high level of reliability required for clinical evaluation or the detection of individual differences (Remmers, Gage, & Rummel, 1965).

2.2.2. Diaries

Parents (usually the mothers, but often assisted by a family member or hired babysitter) were asked to keep a pre-formatted baby diary for three consecutive days, centered on the day of actigraph recording. Each line (row) of the form contained columns to enter the infant’s activity, as well as location and position (for example, lying in playpen in living room), identity of the people present, and the time of day at which any of these variables changed (hence, beginning a new line). After completion of the diary and actigraph recording, the researcher returned to the home to gather the materials and to conduct a semi-structured interview with the mother (and sometimes the father), which included a review of the diary for any entries needing completion or explanation. The purpose of the diary was two-fold: to provide a contextual understanding of sleep patterns, and to allow hand-correction of any artifacts in the actigraph recording (Meltzer, Montgomery-Downs, Insana, et al., 2012), as described above.

The diaries were converted to a computer database consisting of variables representing key elements (for example: sleep/wake, feeding/not) for sequential 1-minute intervals. In order to standardize calculations from reports that varied in the amount of useable data, a 24-hour segment beginning at 7am was selected from each record for analysis. From this database of 1440 consecutive 1-minute intervals per child, we derived five standard measures commonly used in sleep research: total sleep, longest single sleep episode, longest single wake episode, number of sleep episodes, and percent of sleep during nighttime (7pm to 7am).

2.2.3. Statistical tests

Data analysis was carried out with parametric procedures (t-tests, two-tailed) when distributions were indistinguishable from normal by the Kolmogorov-Smirnov test, and the mean, standard deviation, and effect size (Cohen’s d) are reported in the tables; otherwise Wilcoxon’s non-parametric procedure was used, and the descriptive statistics given are the median and range.

3. RESULTS

3.1. Standard Sleep Characteristics

Five common sleep measures are presented in Table 2, estimated by both the diary and actigraph methods. Results from both procedures indicate that the Dutch infants typically slept more than the U.S. infants (Total Sleep), a difference averaging about 1.67 hours according to the more precise actigraph measure. The effect size is large. The greater Dutch sleep was not as fully concentrated at night (Percent Night Sleep), and relatedly the difference in Total Sleep is due almost entirely to daytime sleep (Daytime Sleep), which is apparently caused by a higher number of daytime naps for the Dutch infants (Number of daytime sleep episodes), but not their duration (Average length of daytime sleep episode). The number of nighttime sleep episodes did not differ significantly between the two groups. The Longest Sleep Episode for Dutch infants, however, was nearly an hour longer than for their U.S. counterparts (though the difference is not significant). In contrast, the Longest Wake Episode for Dutch babies was significantly shorter than for the U.S. babies. The difference in when the infants slept is not the result of being put down to bed in the evening at different times (Mdn_NL = 7:12 pm, Mdn_US = 7:30 pm, ns), or when they fell asleep in the evening (12 and 6 minutes later, ns), began their longest sleep (Mdn_NL = 10:42 pm, Mdn_US = 12:20 am, ns), or began their longest wake episode (Mdn_NL = 10:30 am, Mdn_US = 12:30 pm, ns).

Table 2.

Infant Sleep Characteristics (per 24 hours)

Measure	Method	NL	US	Difference	Effect Size (d)	Test Statistic	p
		Mean (SD) or Median (range)	Mean (SD) or Median (range)
Total Sleep (h)
	Diary	13.92 (1.54)	12.42 (1.96)	1.51	0.89	t = 2.03	.05
	Actigraph	13.63 (1.92)	11.96 (1.43)	1.67	1.00	t = 3.41	.001
Number of Sleep Episodes (n)
	Diary	4.31 (2–9)	3.38 (1–6)	0.00	-	z = −1.06	.29
	Actigraph	14.00 (5–22)	12.50 (4–30)	1.50	-	z = 1.00	.32
Longest Sleep Episode (h)
	Diary	10.58 (4.33–12.00)	8.83 (3.25–10.83)	1.75	-	z = 1.91	.06
	Actigraph	4.17 (2.28 – 6.85)	3.19 (1.03 – 7.57)	0.98	-	z = 1.16	.24
Longest Wake Episode (h)
	Diary	3.67 (2.25–10.75)	3.88 (2.83–9.25)	−0.21	-	z = −0.42	.67
	Actigraph	2.98 (0.97 – 5.97)	3.91 (2.08 – 6.50)	−0.93	-	z = −2.01	.04
Percent Night Sleep
	Diary	76 (6.9)	81 (7.2)	−4.5	0.64	t = 2.08	.05
	Actigraph	73.1 (46.1 – 87.1)	79.2 (53.4 – 98.1)	−6.1	-	z = −1.95	.05
Daytime sleep (h)
	actigraph	3.74 (1.29)	2.64 (1.33)	1.10	0.84	t = 2.86	.006
Nighttime sleep (h)
	actigraph	9.73 (6.92–11.17)	9.80 (4.88 – 11.28)	−0.07	-	z = −0.50	.61
Number daytime sleep episodes (raw)
	actigraph	5.5 (2 – 12)	4.0 (1 – 18)	1.5	-	z = 1.37	.17
Number daytime sleep episodes, smoothed 1 min
	actigraph	5.5 (2–12)	3.0 (1–16)	2.15	-	z = −2.01	.04
Average length of daytime episode (h)
	actigraph	0.70 (0.28 – 1.55)	0.56 (0.18 – 1.50)	0.14	-	z = −1.12	.26
Number nighttime sleep episodes
	actigraph	8.0 (3–16)	7.0 (2–16)	1.0	-	z = −0.12	.90
Average length of nighttime episode (h)
	actigraph	1.34 (0.53–3.49)	1.12 (0.41 – 4.83)	0.22	-	z = −.78	.43

3.2. Sleep Architecture

The greater accuracy and detail of the actigraph method permits analysis of sleep architecture, or the internal structuring of sleep stages. There is no significant group difference in the total amount of AS (M_NL = 6.28h, M_US = 6.16h), average length of AS (Mdn_NL = 0.19h, Mdn_US = 0.20h), or longest AS episode (Mdn_NL = 0.83h, Mdn_US = 0.85h), but the Dutch infants had significantly more AS episodes than did the U.S. babies (Mdn_NL = 31.6, Mdn_US = 28.8, z = 1.95, p = .05). The Dutch infants had slightly more and longer average episodes of QS (Mdn_NL = 23.0, 0.35h, Mdn_US = 20.0, 0.28h, both ns), which cumulatively resulted in almost two hours greater total amount of QS (M_NL = 7.36, M_US = 5.50, t = 2.77, p = .01, Cohen’s d = 0.81). Overall, the Dutch infants spent proportionally more of their sleep in QS (53.1%) than did the U.S. infants (46.0%), although the difference is statistically marginal (.09).

As mentioned, there are only minimal group differences in when the infants were typically put to bed and fell asleep. The timing of the longest QS episode, however, is significantly different, as it occurred almost immediately in the evening sleep for the Dutch, and two hours later among the U.S. infants ( Mdn_NL = 7:30 pm, Mdn_US = 9:30 pm, z = − 2.56, p = .01). The longest episode of AS started toward the end of the night (4:00 am) for most infants in both groups.

3.3. Examination of potential confounds

To examine the possible influence of demographic differences (Table 1) on the cultural comparison of sleep, key analyses were repeated using each of these variables as a covariate control prior to the test of group differences. The pattern of contrast was not altered, and the differences were actually increased for some measures, for example Total Sleep (by actigraph, adjusted for maternal age, maternal education, and maternal employment, M_NL = 12.97, M_US = 12.01, difference = 0.96 hours, t(38) = 4.87, p < .0001, Cohen’s d = 1.57). Additional analyses indicate no significant effect of weekday vs. weekend recordings, or the various childcare parameters in Table 1.

Statistical adjustments for hours of daylight (United States Navy, 2007), and for season of the year, show little influence on our sleep results and does not alter any of the conclusions, a finding in agreement with other published reports (Hense et al.., 2011; Park, Kripke, & Cole, 2007).

The sleep-scoring algorithm for the actigraph (Sadeh et al., 1995) was developed with American infants, so we examined the possibility that the dividing line between AS and QS might be set too high for a Dutch sample, yielding some minutes now scored as QS to lie just below the threshold and which would, if revalidated with a Dutch sample, end up being categorized as AS. This proved not to be the case, as the Dutch infants in fact were significantly less active during QS than their U.S. counterparts.

Relatedly, the Dutch infants were less active while awake compared to the U.S. infants (M_NL = 220.2, M_US = 235.9, z = −2.03, p = .04). They were also less variable, as a group, in their activity level (SD_NL = 85.0, SD_US = 93.4, z = −2.43, p = .02).

4. DISCUSSION

Two findings from the present study are noteworthy, and they largely confirm our initial expectations. Regarding the duration of infant sleep (Total Sleep), this study is the first to report significant and substantial differences between two cultural groups using objective actigraph measurement. Although the actigraph and diary methods yield different absolute results, for reasons outlined earlier in this report, the group difference is estimated to be about the same by the two methods -- 1.51 and 1.67 hours -- figures that are comparable to other reports for these cultural groups (Huitron ´ et al., 2019; Super et al., 1996). In global perspective (Mindell, et al.., 2010), the Dutch and U.S. figures appear, respectively, to be among the highest and lowest levels reported among Western samples, although this comparison should be viewed with caution due to age differences in the samples and previously noted methodological issues.

It is interesting to note that the group difference in Total Sleep results almost exclusively from daytime sleep – both the Dutch and U.S. babies average 9.75 hours per night. We can only speculate on the cause of this pattern: Perhaps with a calmer and less physically active day, as demonstrated, the Dutch infants have more opportunity to nap, while the U.S. infants are more stimulated by their caretakers and mechanical toys (Harkness et al., 2007; Super et al., 1996; van Schaik et al., 2020)

Second, and perhaps of greater significance, our results indicate that the internal architecture of sleep also varies between the two cultural samples, a topic that has not previously been subjected to systematic examination during infancy. The Dutch infants, compared to those in the U.S. sample, were observed to sleep more (both absolutely and proportionally) in the Quiet, rather than the Active phase. In addition, they began their QS earlier in the evening than their U.S. counterparts.

Comparing these findings regarding sleep duration and sleep phases to the established understanding of sleep maturation, it is striking that the current results point in two opposing directions. The shorter duration of total sleep, its greater concentration into the night hours, and the longer maximal periods of wakefulness would, by themselves, suggest greater maturity on the part of the U.S. infants. On the other hand, the greater amount of Quiet Sleep, the appearance of QS earlier in the night, and the possibly longer maximal periods of sleep, would indicate greater maturity on the part of the Dutch infants. It seems likely that these two, divergent patterns represent adaptation to different, culturally organized environments, and the coherence of measures of maturation in any one cultural setting is in part an artifact of external structuring in the niche.

4.2. Ethnographic concordance

The behavioral results reported here are consistent with our ethnographic understanding of the two cultural places. The settings, customs, and parental ethnotheories that form the infants’ developmental niches (Super & Harkness, 1986) in the two samples appear quite similar to our earlier research in the northeast U.S. and the randstaat area of the Netherlands, especially with regard to the thematic emphasis on restful regularity vs. developmental stimulation (Harkness & Super, 1999; Harkness et al., 2007; Super et al.,1996; van Schaik et al., 2020). The contrasting picture of infant care is also consistent with the historic record (Child, 1831/1992; Rebelsky & Dorman, 1973).

The broad consistency of findings across related samples and across generational time suggests that although the present report concerns a small number of infants in two circumscribed places, the families in each site share with others around them, and with generations before them, a common cultural model of early development, sustained through historical meaning and contemporary interaction with people, media, and institutions. The current literature on sleep is consistent in concluding that environmental causes of variation are more influential than genetic ones (Fisher, van Jaarsveld, Llewellyn, et al., 2012); each genetic study, however, has been carried out in only one cultural environment. Cultural differences, as the present study demonstrates, can be lasting and substantial; thus the true difference in environmental vs. genetic influence may be even greater than the current literature estimates.

4.3. Methodological observation

The two methods of data collection – actigraph and diary – yield unexpectedly close results for Total Sleep, given the literature on methodological discrepancies (Sadeh, Acebo, Seifer, & Aytur, 1995). Our estimates of Total Sleep from the two methods were within half an hour of each other in both samples; the resulting group difference calculations (from actigraph vs. diary) were less than 10 minutes apart. Earlier methodology assessments have found an average discrepancy of 1.75 hours (with older subjects, 4 to 7 years: Werner, et al., 2008). In addition to the age difference of subjects here and in Werner et al.., our findings might be influenced by the method of averaging across children, within group, rather than analyzing individual measures. As with other reports (Hall, Liva, Moynihan, et al., 2015; Sadeh, et al.., 1995; Werner et al.., 2008), the invisibility to parents of some night waking produced a substantial discrepancy here between the diary and actigraph estimates for Longest Sleep Episode.

4.4. Implications for healthy development

Do these findings have implications for long-term development? One could argue that the consequences should be minimal because the mutually influencing systems of physiology and behavior become coordinated during development, ultimately approximating homeostasis (Susman, 1998). In particular, adaptation-level theory (Helson, 1964) would suggest that sleep-related systems adapt to a typical level of activation and, other things being equal, resolve to a pattern not associated with increased risk.

Alternatively, the differences in sleep development found here might persist in both pattern and effect. Nearly 40% of U.S. babies are judged by their parents to get less sleep than they need (National Sleep Foundation, 2004), and the present results suggest they may in fact get less sleep than their Dutch counterparts. It has long been an axiom of developmental theory that influences early in life, especially those operating during “sensitive periods” of organization, are particularly profound and lasting. More specifically, there is an emerging consensus that bio-behavioral coordination established during early infancy is influential for subsequent patterns of self-regulation (Hertzman, 2012; Meaney, Aitken, Bhatnagar, et al., 1988; Susman, 1998). This perspective would suggest that infants experiencing higher levels of arousal and less sleep might have different adaptive responses to social and environmental stressors than might other infants.

The literature briefly reviewed in the introduction regarding the consequences of minimal sleep lend weight to this latter position, as does our observed group difference in Quiet Sleep and Active Sleep. Human growth hormone, for example, is secreted during Quiet Sleep (Carbone, 1985), and the substantially greater amount of Quiet Sleep among the Dutch infants may contribute to the current status of the Dutch as the tallest population in the world (American Association for the Advancement of Science, 2015). Acknowledging a cultural basis of divergence during infancy, however, substantially complicates the interpretation of any longitudinal correlation derived from monocultural studies. It also renders problematic efforts to pluck out a single custom, such as more daytime napping, and apply it elsewhere. The child-rearing practices described here are only one aspect of more complex and pervasive differences between Dutch and U.S. life. The environmental contrast leading to infant sleep differences may simply start developmental trajectories in which continued environmental differences sustain and elaborate the initial divergence. Continuities in the cultural environment cannot easily be disentangled from long-term consequences of an early developmental difference.

4.5. Limitations and future directions

This study is limited by the relatively small number of participants, and by the single-day measurement of sleep. Some protection, however, is provided for each of these. With regard to the sample size, the populations are closely defined and well-studied, and there are many points of concordance between findings here and previous research with other members of the same sub-populations; the new findings – in accuracy and detail – are consistent with earlier results. Secondly, infants are well-known for their variability in nightly sleep, so the unusually short actigraph recording is potentially a limitation. If one recognizes, however, that the object of assessment is not each child’s typical sleep, but rather the typical sleep of infants in a particular cultural place, it is easier to see why the several reports from these sub-populations yield such similar findings: they are reliable assessments of infants’ sleep in each cultural community.

Given the stability of this comparison, and the new detail about differential patterns of maturity, a number of new questions arise. Are there other aspects of infant physiology that similarly diverge in the two groups? Is there continuity over several years in the divergence? Ultimately, of course, does it make a difference? Only more detailed study, over time, can address these questions.

5. CONCLUSION

Although the development of sleep in infancy is fundamentally a maturational process, it is equally a culturally organized behavior. The present results are but one instance of the postulate that “maturity” in any domain – physiology, motor skill, cognition, affect – is not absolute but rather patterned in reflection of the developing child’s adaptation to the specific demands and affordances of the niche. An empirically informed respect for the range of culturally induced patterns can deepen our understanding of the nature of early human development, and support our efforts to assist parents in raising healthy children.

Highlights.

• Many Dutch parents emphasize rest and regularity for their infants.

• 8-month-old Dutch babies slept 1.67 hours more than U.S. babies.

• They also showed a more mature pattern of Quiet Sleep.

• The differences correspond to parental beliefs and practices.

• The results highlight the importance of cultural context.