Differences in insomnia-related factors between working-age and older adults: A comparison of nocturnal and daytime symptoms

Abstract

Objective

This study aimed to (1) clarify the characteristics of nocturnal and daytime symptoms among working-age and older adults and (2) classify insomnia types within each age group and examine the characteristics associated with each type.

Methods

An online survey was conducted, and data from 1655 participants were analyzed. Participants were categorized into working-age adults (30–64 years) and older adults (≥65 years). Based on the presence or absence of nocturnal symptoms and daytime dysfunction according to the International Classification of Sleep Disorders, Third Edition criteria, participants were classified as normal sleepers, having subsyndromal insomnia, or having insomnia syndrome. Nocturnal and daytime symptoms were assessed using validated self-reported measures.

Results

Compared with older adults, working-age adults showed significantly worse scores for all nocturnal and daytime symptoms. In working-age adults, subsyndromal insomnia was associated with sleep-wake characteristics, whereas insomnia syndrome was strongly associated with sleep debt, fatigue, and anxiety. Conversely, insomnia syndrome was primarily associated with sleep debt and fatigue among older adults. Notably, fatigue showed a greater discriminative ability than daytime sleepiness in distinguishing insomnia types across age groups.

Conclusion

The nocturnal and daytime profiles associated with sleep problems differed between working-age and older adults. These findings suggest that age-specific assessment and treatment strategies are warranted and that fatigue may be a particularly important indicator when evaluating daytime dysfunction related to insomnia.

Graphical abstract

Highlights

• •Different insomnia types showed distinct symptom profiles.
• •Age-specific assessment and intervention strategies for insomnia are important.
• •Focusing on fatigue may enhance evaluation of the daytime impact of sleep problems.

1. Introduction

Insomnia has been widely studied because of its close association with mental, physical, and social well-being. Insomnia is generally characterized by nocturnal symptoms, such as difficulty in initiating and maintaining sleep, and daytime dysfunction, including fatigue and excessive daytime sleepiness. In Japan, approximately 13.5% of the general population is estimated to have subsyndromal insomnia, defined as insomnia symptoms without accompanying daytime dysfunction, whereas approximately 3.8% meet the criteria for insomnia syndrome, characterized by insomnia symptoms with daytime dysfunction [1]. A recent meta-analysis reported a global prevalence of insomnia of 16.2% [2], and the prevalence in Asian countries has been estimated at 11.6% (95% confidence interval [CI]: 6.4–20.2) [3]. The prevalence of insomnia has been shown to increase from the late 30s onward [4].

Insomnia is also associated with a wide range of adverse daytime effects. A large cohort study [5] demonstrated that individuals with insomnia syndrome experience a higher burden of daytime functional impairments, such as dizziness, fatigue, and excessive sleepiness, than those without insomnia. Additionally, insomnia symptoms are associated with mental health issues, including anxiety and depression. Other studies have reported that insomnia is linked to reduced work performance [6,7] and an increased risk of suicide-related behaviors [8].

Sleep characteristics differ across age groups. In Japan, studies on working-age adults, particularly those in their 30s to 50s, have shown that approximately 42–51% sleep fewer than 6 h/night, with work demands, household responsibilities, and childcare identified as major contributors to insufficient sleep [9]. Short sleep duration in this population is also associated with an increased risk of lifestyle-related diseases [10]. Furthermore, attempts to compensate for weekday sleep loss by sleeping longer on weekends may lead to social jetlag, which has been reported to be prevalent among working-age adults in Japan [11]. Taken together, insufficient sleep (sleep debt) and its consequences appear to be particularly prominent among working-age adults.

In contrast, older adults exhibit sleep characteristics that differ from those observed in younger populations. Physiologically, the amount of sleep required tends to decrease slightly with age [12,13], and age-related changes in circadian rhythms, such as reduced differentiation between day and night and an advance in the circadian phase, have been reported [14,15]. The prevalence of subsyndromal insomnia among older adults is estimated to be 30–48% [16]. Insomnia later in life has been associated with an increased risk of Alzheimer's disease [17], higher mortality related to daytime impairment [18], and an elevated risk of mortality associated with long sleep duration or prolonged bedtime [19,20]. These findings suggest that the clinical implications of insomnia may differ substantially between working-age and older adults.

Moreover, previous studies have indicated that different insomnia subtypes are associated with distinct clinical outcomes. A longitudinal cohort study [21] demonstrated that subsyndromal insomnia has an approximately two-fold higher remission rate than does insomnia syndrome. Additionally, individuals with insomnia syndrome incur higher insomnia-related healthcare costs, experience greater work absenteeism, and have reduced productivity than those with subsyndromal insomnia [22]. Furthermore, Hara et al. [23] found that individuals with insomnia syndrome exhibit poorer sleep hygiene practices than those with subsyndromal insomnia or normal sleepers. These findings indicate that subsyndromal insomnia and insomnia syndrome may represent distinct clinical states, rather than differing solely in severity.

Despite growing evidence that sleep characteristics and the consequences of insomnia differ by age group, few studies have directly compared insomnia-related nocturnal and daytime symptoms between working-age and older adults. Moreover, to the best of our knowledge, no previous study has examined the differences in sleep problems and daytime symptoms between subsyndromal insomnia and insomnia syndrome while explicitly distinguishing between these two age groups. Therefore, we aimed to: (1) clarify the characteristics of nocturnal and daytime symptoms among working-age and older adults and (2) classify insomnia types within each age group and examine the characteristics associated with each type.

2. Methods

2.1. Participants

The data analyzed in this study were collected in December 2020 through an online survey. Participants were recruited by Cross Marketing Inc., a Japanese online research company that maintains a panel of approximately 5.4 million registered respondents nationwide. In total, 2808 individuals participated in the survey. Responses were excluded if participants reported identical bedtimes and wake times, showed negative scores on the Sleep Debt Index (SDI) [24], reported a total sleep time of 15 h or longer, or provided incomplete or implausible sleep-related data. Shift workers were excluded from the analysis. After applying these exclusion criteria, data from 1655 participants were retained for analysis (Fig. 1). Participants were classified into two age groups: working-age adults (<65 years) and older adults (≥65 years).

Fig. 1.

Participants flow chart.

2.2. Measures

2.2.1. Demographic variables

Demographic data included age, sex, employment status (full-time, part-time, self-employed, unemployed, or other), and engagement in irregular shift work involving nighttime hours.

2.2.2. Insomnia criterion

Based on the diagnostic criteria of the International Classification of Sleep Disorders, Third Edition (ICSD-3) [25], participants were asked whether nocturnal insomnia symptoms (difficulty initiating sleep, difficulty maintaining sleep, and early morning awakening) occurred at least three times per week and persisted for at least 3 months. Participants who reported insomnia symptoms lasting for 3 months or longer were additionally asked whether these symptoms caused daytime dysfunction.

2.2.3. Nocturnal symptoms

Japanese version of Athens Insomnia Scale (AIS) [26]; the scale is an eight-item self-report questionnaire rated on a four-point Likert scale, designed to assess the severity of insomnia symptoms. A total score of 6 or higher indicates the presence of clinically relevant insomnia symptoms. Higher scores reflect greater insomnia severity [26].

Sleep Debt Index (SDI) [24]; the scale consists of three items assessing weekday sleep duration, weekend sleep duration, and ideal sleep duration. Sleep debt is calculated as the difference between ideal sleep duration and the weekly average of actual sleep duration [24]. Higher scores indicate greater sleep debt. In the present study, participants with an SDI score of 1 h or greater were classified as having sleep debt, which is consistent with previous research [27].

Ultra-short version of the Munich Chronotype Questionnaire (μMCTQ) [28]; the scale is a six-item questionnaire derived from the original MCTQ [29]. In this study, the midpoint of sleep on free days, sleep-corrected (MSFsc), was used as an indicator of the circadian phase. Higher MSFsc values indicate a delayed sleep-wake phase. For analysis, participants were classified into two groups based on the median MSFsc value: those with an advanced sleep-wake rhythm (≤median) and those with a delayed sleep-wake rhythm (>median).

2.2.4. Daytime symptoms

Japanese version of the Epworth Sleepiness Scale (JESS) [30]; daytime sleepiness was assessed using this scale. The JESS consists of eight items rated on a 4-point Likert scale, with higher scores indicating greater daytime sleepiness. In the present study, participants scoring at or above the mean value were classified as having daytime sleepiness for statistical analysis.

Japanese version of the Brief Fatigue Inventory (BFI) [31]; fatigue was assessed using this scale. The BFI consists of 10 items rated on an 11-point scale, with higher scores indicating greater fatigue severity. Participants with scores above the sample mean were classified as having fatigue.

Japanese version of Hospital Anxiety and Depression Scale (HADS) [32]; anxiety and depressive symptoms were assessed using this scale, a 14-item questionnaire rated on a 4-point Likert scale. Total HADS scores, as well as anxiety and depression subscale scores, were included in the analyses. Higher scores indicate more severe symptoms. A cut-off score of 8 was applied to both the anxiety and depression subscales to identify clinically relevant symptoms, consistent with established criteria.

2.3. Procedure

Following the classification approach proposed by Morin et al. [21], participants were categorized into three insomnia types based on their responses regarding insomnia criterion, according to the ICSD-3 criteria: insomnia syndrome, subsyndromal insomnia, and normal sleepers. Insomnia syndrome was defined as the presence of at least one nocturnal insomnia symptom (difficulty initiating sleep, difficulty maintaining sleep, or early morning awakening) occurring at least three times per week for 3 months or longer, accompanied by self-reported daytime dysfunction attributable to these symptoms. Subsyndromal insomnia was defined as the presence of nocturnal insomnia symptoms occurring at least three times per week for 3 months or longer without accompanying daytime dysfunction. Normal sleepers were defined as those who reported neither clinically relevant nocturnal insomnia symptoms nor daytime dysfunction.

2.4. Statistical analysis

Participants were first classified into working-age adults (<65 years) and older adults (≥65 years). Independent-sample t-tests were conducted to compare the scales for nocturnal and daytime symptoms between the two age groups. Second, within each age group, participants were classified into three insomnia types (normal sleepers, subsyndromal insomnia, and insomnia syndrome). One-way analyses of variance (ANOVAs) were conducted separately for each age group, with insomnia type as the independent variable and all scales as the dependent variables.

Finally, hierarchical logistic regression analyses were performed to examine the risk factors associated with insomnia type in each age group. The independent variables included all scales for nocturnal and daytime symptoms, and the dependent variables were defined as pairwise comparisons between insomnia types (normal sleepers vs. subsyndromal insomnia, normal sleepers vs. insomnia syndrome, and subsyndromal insomnia vs. insomnia syndrome). Sex was included as a covariate in all models. Odds ratios (ORs) with 95% CIs were calculated to identify associated risk factors. For measures with established cut-off values, these cut-offs were applied, while for measures without predefined cut-offs, dichotomization was performed using mean or median scores.

All statistical analyses were conducted using IBM SPSS Statistics version 25 (IBM Corp., Armonk, NY, USA).

2.5. Ethical considerations

This study was approved by the Ethics Committee of Tokyo Kasei University (approval ID: Ita-2020-15; October 28, 2020). All participants provided informed consent before participation.

3. Results

3.1. Differences in nocturnal sleep and daytime functioning between age groups

The participants’ characteristics are summarized in Table 1. Regarding the distribution of insomnia types within each age group, 19.5% of working-age adults were classified as having insomnia syndrome and 10.5% as having subsyndromal insomnia. Among older adults, 12.8% were classified as having insomnia syndrome and 10.0% as having subsyndromal insomnia.

Table 1.

Demographic characteristics and rates of Insomnia types.

	Total		Working-age adults		Older adults		t value or χ2 value	p-value	post hoc multiple comparison or Cramer's V
	n = 1655		n = 1106		n = 549
	Mean	%	Mean	%	Mean	%
Age (SD)	58.79	(9.87)	53.30	(6.78)	69.87	(4.19)	−52.46	<0.01	Older > Working-age
Sex							30.91	<0.01	0.14
Male	1178	71.2%	739	66.8%	439	80.0%			Older > Working-age
Femal	477	28.8%	367	33.2%	110	20.0%			Working-age > Older
Occupational status							493.85	<0.01	0.55
Regular employment	825	49.8%	723	65.4%	102	18.6%			Working-age > Older
Non-regular employment	214	12.9%	145	13.1%	69	12.6%			-
Self-employed	179	10.8%	111	10.0%	68	12.4%			-
Unemployed	371	22.4%	83	7.5%	288	52.5%			Older > Working-age
Other	66	4.0%	44	4.0%	22	4.0%			-
Insomnia Type							12.80	<0.01	0.09
Normal Sleeper	1197	72.3%	773	70.0%	424	77.2%			Older > Working-age
Subsyndromal Insomnia	171	10.3%	116	10.5%	55	10.0%			-
Insomnia Syndrome	287	17.3%	217	19.5%	70	12.8%			Working-age > Older

Note. Working-age adults refers to participants aged 30–64 years; Older adults refers to participants aged 65 years and older.

The results of the nocturnal sleep and daytime functioning measures in each age group are presented in Table 2 t-tests revealed that working-age adults had significantly worse scores than did older adults for all nocturnal and daytime symptoms (p values < 0.05). Additionally, ANOVAs were conducted to examine the differences between nocturnal and daytime symptoms within each age group. Among working-age adults, scores on all scales worsened sequentially from normal sleepers to subsyndromal insomnia and to insomnia syndrome, except for JESS (p values < 0.05; Table 3). For JESS scores, a significant difference was observed only between normal sleepers and those with insomnia syndrome.

Table 2.

Comparison of each scales between age groups.

	Total		Working-age adults		Older adults		t value	p-value	Post hoc tests
	n = 1655		n = 1106		n = 549
	Mean	SD	Mean	SD	Mean	SD
AIS	5.37	(4.18)	5.81	(4.34)	4.47	(3.67)	6.59	<0.01	Working-age > Older
SDI	0.87	(0.84)	1.00	(0.86)	0.61	(0.71)	9.52	<0.01	Working-age > Older
MSFsc	1.88	(3.18)	2.14	(3.17)	1.34	(3.14)	4.88	<0.01	Working-age > Older
JESS	7.68	(5.03)	8.10	(5.16)	6.84	(4.65)	5.01	<0.01	Working-age > Older
BFI-J	2.60	(2.04)	2.94	(2.12)	1.90	(1.67)	10.92	<0.01	Working-age > Older
HADS
Total	13.50	(6.43)	14.78	(6.59)	10.91	(5.23)	12.97	<0.01	Working-age > Older
Anxiety	5.04	(3.83)	5.76	(4.02)	3.57	(2.91)	12.63	<0.01	Working-age > Older
Depression	8.46	(3.67)	9.02	(3.68)	7.34	(3.38)	9.24	<0.01	Working-age > Older

Note. SD, standard deviation; AIS, Athens Insomnia scale; BFI-J, Japanese version of the Brief Fatigue Inventory; HADS, Hospital Anxiety and Depression Scale; JESS, Japanese version of the Epworth Sleepiness Scale; MSFsc, Midpoint of Sleep on Free days, sleep-corrected; SDI, Sleep Debt Index.

Table 3.

Comparison of each scale between insomnia type in working-age adults.

	Normal Sleepers		Subsyndromal Insomnia		Insomnia Syndrome		F value or χ2 value	p-value	Effect size η2p	Post hoc tests
	n = 773		n = 116		n = 217
	Mean	SD	Mean	SD	Mean	SD
Age (years)	53.38	(6.76)	53.10	(6.59)	53.11	(6.99)	0.19	0.83	0.000	-
Gender (men/%)	519	67.1%	78	67.2%	142	65.4%	0.23	0.89	-	-
AIS	4.50	(3.53)	6.47	(3.25)	10.15	(4.56)	196.24	<0.01	0.262	Normal < Subsyndromal < Syndrome
SDI	0.86	(0.77)	1.09	(0.95)	1.44	(0.96)	42.90	<0.01	0.072	Normal < Subsyndromal < Syndrome
MSFsc	2.22	(3.18)	1.75	(2.92)	2.09	(3.25)	1.13	0.324	0.002	-
JESS	7.90	(5.01)	7.44	(4.98)	9.17	(5.63)	6.27	<0.01	0.011	Normal, Subsyndromal < Syndrome
BFI-J	2.49	(1.90)	3.08	(2.06)	4.47	(2.18)	85.47	<0.01	0.134	Normal < Subsyndromal < Syndrome
HADS
Total	13.46	(5.97)	15.42	(5.59)	19.15	(7.24)	71.93	<0.01	0.115	Normal < Subsyndromal < Syndrome
Anxiety	4.91	(3.49)	6.09	(3.50)	8.64	(4.62)	84.66	<0.01	0.133	Normal < Subsyndromal < Syndrome
Depression	8.56	(3.61)	9.34	(3.44)	10.51	(3.64)	25.46	<0.01	0.044	Normal < Subsyndromal < Syndrome

Note. Normal: Normal Sleeper, Subsyndromal: Subsyndromal Insomnia, Syndrome:Insomnia Syndrome.SD, standard deviation; AIS, Athens Insomnia scale; BFI-J, Japanese version of the Brief Fatigue Inventory; HADS, Hospital Anxiety and Depression Scale; JESS, Japanese version of the Epworth Sleepiness Scale; MSFsc, Midpoint of Sleep on Free days, sleep-corrected; SDI, Sleep Debt Index.

Among older adults, AIS and SDI scores worsened in the following order: normal sleepers, subsyndromal insomnia, and insomnia syndrome (p values < 0.05; Table 4). Regarding the JESS score, a significant difference was observed only between normal sleepers and those with insomnia syndrome (p < 0.05). Regarding total score, anxiety, and depression subscale scores for HADS, participants with insomnia syndrome had significantly higher scores than the other two groups (p values < 0.05).

Table 4.

Comparison of each scale between insomnia type in older adults.

	Normal Sleepers		Subsyndromal Insomnia		Insomnia Syndrome		F value or χ2 value	p-value	Effect size η2p	Post hoc tests
	n = 424		n = 55		n = 70
	Mean	SD	Mean	SD	Mean	SD
Age (years)	69.83	(4.16)	69.98	(4.00)	69.97	(4.56)	0.06	0.95	0.000	-
Gender (men/%)	336	79.2%	42	76.4%	61	87.1%	2.833	0.24	-	-
AIS	3.59	(2.94)	6.25	(3.26)	8.43	(4.73)	75.87	<0.01	0.217	Normal < Subsyndromal < Syndrome
SDI	0.48	(0.61)	0.91	(0.77)	1.17	(0.88)	38.17	<0.01	0.123	Normal < Subsyndromal < Syndrome
MSFsc	1.42	(3.11)	0.96	(3.20)	1.18	(3.25)	0.62	0.54	0.002	-
JESS	6.39	(4.38)	7.51	(5.17)	9.03	(5.18)	10.68	<0.01	0.038	Normal < Syndrome
BFI-J	1.67	(1.46)	1.91	(1.54)	3.30	(2.22)	31.90	<0.01	0.105	Normal, Subsyndromal < Syndrome
HADS
Total	10.39	(4.94)	11.04	(4.65)	14.01	(6.24)	15.22	<0.01	0.053	Normal, Subsyndromal < Syndrome
Anxiety	3.25	(2.69)	3.67	(2.58)	5.41	(3.72)	17.55	<0.01	0.060	Normal, Subsyndromal < Syndrome
Depression	7.13	(3.34)	7.36	(2.94)	8.60	(3.70)	5.77	<0.01	0.021	Normal, Subsyndromal < Syndrome

Note. Normal: Normal Sleeper, Subsyndromal: Subsyndromal Insomnia, Syndrome:Insomnia Syndrome.SD, standard deviation; AIS, Athens Insomnia scale; BFI-J, Japanese version of the Brief Fatigue Inventory; HADS, Hospital Anxiety and Depression Scale; JESS, Japanese version of the Epworth Sleepiness Scale; MSFsc, Midpoint of Sleep on Free days, sleep-corrected; SDI, Sleep Debt Index.

3.2. Factors associated with insomnia type in each age group

The results of the hierarchical logistic regression analyses are presented in Table 5, Table 6. Among working-age adults, compared with normal sleepers, subsyndromal insomnia was significantly associated with higher AIS scores (OR [95%CI] = 3.00 [1.94–4.63]) and lower MSFsc values (OR = 0.64 [0.42–0.95]; Table 5). A comparison of normal sleepers with those with insomnia syndrome revealed significant associations of insomnia syndrome with higher AIS (OR = 8.44 [5.43–13.12]), SDI (OR = 1.87 [1.29–2.71]), BFI (OR = 1.62 [1.05–2.49]), and HADS anxiety (OR = 2.30 [1.56–3.39]) scores. Furthermore, a comparison of subsyndromal insomnia with insomnia syndrome revealed significant associations of insomnia syndrome with higher AIS (OR = 2.96 [1.64–5.33]), JESS (OR = 1.70 [1.02–2.83]), and BFI (OR = 2.12 [1.22–3.70]) scores.

Table 5.

Results of logistic regression analyses in working-age adults.

	STEP 1		STEP 2		STEP 3
	OR	95% CI	OR	95% CI	OR	95% CI
AIS
Normal Sleeper	Ref		Ref		Ref
Subsyndromal Insomnia	3.00∗	(1.99-4.53)	3.00∗	(1.94-4.63)	3.00∗	(1.94-4.63)
Insomnia Syndrome	11.47∗	(7.51-17.54)	8.44∗	(5.43-13.12)	8.44∗	(5.43-13.12)
Subsyndromal Insomnia	Ref		Ref		Ref
Insomnia Syndrome	3.66∗	(2.10-6.39)	2.96∗	(1.64-5.33)	2.96∗	(1.64-5.33)
SDI
Normal Sleeper	Ref		Ref		Ref
Subsyndromal Insomnia	1.22	(0.81-1.84)	1.23	(0.81-1.86)	1.23	(0.81-1.86)
Insomnia Syndrome	2.17∗	(1.52-3.10)	1.87∗	(1.29-2.71)	1.87∗	(1.29-2.71)
Subsyndromal Insomnia	Ref		Ref		Ref
Insomnia Syndrome	1.73∗	(1.05-2.84)	1.51	(0.89-2.54)	1.51	(0.89-2.54)
MSFsc
Normal Sleeper	Ref		Ref		Ref
Subsyndromal Insomnia	0.63∗	(0.42-0.94)	0.64∗	(0.42-0.95)	0.64∗	(0.42-0.95)
Insomnia Syndrome	0.94	(0.66-1.32)	0.91	(0.64-1.30)	0.91	(0.64-1.30)
Subsyndromal Insomnia	Ref		Ref		Ref
Insomnia Syndrome	1.53	(0.95-2.47)	1.41	(0.86-2.33)	1.41	(0.86-2.33)
JESS
Normal Sleeper	Ref		Ref		Ref
Subsyndromal Insomnia	-	-	0.67	(0.44-1.02)	0.67	(0.44-1.02)
Insomnia Syndrome	-	-	1.06	(0.73-1.53)	1.06	(0.73-1.53)
Subsyndromal Insomnia	Ref		Ref		Ref
Insomnia Syndrome	-	-	1.70∗	(1.02-2.83)	1.70∗	(1.02-2.83)
BFI-J
Normal Sleeper	Ref		Ref		Ref
Subsyndromal Insomnia	-	-	0.84	(0.52-1.35)	0.84	(0.52-1.35)
Insomnia Syndrome	-	-	1.62∗	(1.05-2.49)	1.62∗	(1.05-2.49)
Subsyndromal Insomnia	Ref		Ref		Ref
Insomnia Syndrome	-	-	2.12∗	(1.22-3.70)	2.12∗	(1.22-3.70)
HADS anxiety
Normal Sleeper	Ref		Ref		Ref
Subsyndromal Insomnia	-	-	1.50	(0.92-2.45)	1.50	(0.92-2.45)
Insomnia Syndrome	-	-	2.30∗	(1.56-3.39)	2.30∗	(1.56-3.39)
Subsyndromal Insomnia	Ref		Ref		Ref
Insomnia Syndrome	-	-	1.45	(0.83-2.53)	1.45	(0.83-2.53)
HADS depression
Normal Sleeper	Ref		Ref		Ref
Subsyndromal Insomnia	-	-	1.22	(0.76-1.94)	1.22	(0.76-1.94)
Insomnia Syndrome	-	-	1.20	(0.77-1.88)	1.20	(0.77-1.88)
Subsyndromal Insomnia	Ref		Ref		Ref
Insomnia Syndrome	-	-	1.09	(0.59-2.00)	1.09	(0.59-2.00)

Note. OR, odds ratio; CI, confidence interval; Ref, reference; AIS, Athens Insomnia scale; BFI-J, Japanese version of the Brief Fatigue Inventory; HADS, Hospital Anxiety and Depression Scale; JESS, Japanese version of the Epworth Sleepiness Scale; MSFsc, Midpoint of Sleep on Free days, sleep-corrected; SDI, Sleep Debt Index.STEP 1 included nighttime symptoms (AIS, SDI, and MSFsc),STEP 2 added daytime symptoms(JESS,BFI-J, and HADS) and STEP 3 additionaly adjusted for gender.∗p < 0.05.

Table 6.

Results of logistic regression analyses in older adults.

	STEP 1		STEP 2		STEP 3
	OR	95% CI	OR	95% CI	OR	95% CI
AIS
Normal Sleeper	Ref		Ref		Ref
Subsyndromal Insomnia	3.38∗	(1.84-6.22)	3.43∗	(1.81-6.48)	3.43∗	(1.81-6.48)
Insomnia Syndrome	7.30∗	(3.92-13.60)	5.38∗	(2.79-10.37)	5.38∗	(2.79-10.37)
Subsyndromal Insomnia	Ref		Ref		Ref
Insomnia Syndrome	2.39∗	(1.09-5.26)	1.61	(0.67-3.87)	1.61	(0.67-3.87)
SDI
Normal Sleeper	Ref		Ref		Ref
Subsyndromal Insomnia	2.03∗	(1.09-3.76)	2.01∗	(1.08-3.74)	2.01∗	(1.08-3.74)
Insomnia Syndrome	3.06∗	(1.69-5.55)	2.75∗	(1.49-5.10)	2.75∗	(1.49-5.10)
Subsyndromal Insomnia	Ref		Ref		Ref
Insomnia Syndrome	1.74	(0.82-3.70)	1.75	(0.79-3.87)	1.75	(0.79-3.87)
MSFsc
Normal Sleeper	Ref		Ref		Ref
Subsyndromal Insomnia	0.71	(0.38-1.31)	0.69	(0.37-1.29)	0.69	(0.37-1.29)
Insomnia Syndrome	0.63	(0.34-1.15)	0.56	(0.30-1.04)	0.56	(0.30-1.04)
Subsyndromal Insomnia	Ref		Ref		Ref
Insomnia Syndrome	0.82	(0.37-1.82)	0.75	(0.33-1.71)	0.75	(0.33-1.71)
JESS
Normal Sleeper	Ref		Ref		Ref
Subsyndromal Insomnia	-	-	1.30	(0.69-2.43)	1.30	(0.69-2.43)
Insomnia Syndrome	-	-	1.32	(0.72-2.43)	1.32	(0.72-2.43)
Subsyndromal Insomnia	Ref		Ref		Ref
Insomnia Syndrome	-	-	1.38	(0.63-3.03)	1.38	(0.63-3.03)
BFI-J
Normal Sleeper	Ref		Ref		Ref
Subsyndromal Insomnia	-	-	0.66	(0.31-1.40)	0.66	(0.31-1.40)
Insomnia Syndrome	-	-	1.86	(0.99-3.50)	1.86	(0.99-3.50)
Subsyndromal Insomnia	Ref		Ref		Ref
Insomnia Syndrome	-	-	3.02∗	(1.30-7.03)	3.02∗	(1.30-7.03)
HADS anxiety
Normal Sleeper	Ref		Ref		Ref
Subsyndromal Insomnia	-	-	1.00	(0.37-2.72)	1.00	(0.37-2.72)
Insomnia Syndrome	-	-	1.40	(0.65-3.01)	1.40	(0.65-3.01)
Subsyndromal Insomnia	Ref		Ref		Ref
Insomnia Syndrome	-	-	1.71	(0.57-5.12)	1.71	(0.57-5.12)
HADS depression
Normal Sleeper	Ref		Ref		Ref
Subsyndromal Insomnia	-	-	1.22	(0.66-2.24)	1.22	(0.66-2.24)
Insomnia Syndrome	-	-	1.36	(0.74-2.53)	1.36	(0.74-2.53)
Subsyndromal Insomnia	Ref		Ref		Ref
Insomnia Syndrome	-	-	0.96	(0.40-2.28)	0.96	(0.40-2.28)

Note. OR, odds ratio; CI, confidence interval; Ref, reference; AIS, Athens Insomnia scale; BFI-J, Japanese version of the Brief Fatigue Inventory; HADS, Hospital Anxiety and Depression Scale; JESS, Japanese version of the Epworth Sleepiness Scale; MSFsc, Midpoint of Sleep on Free days, sleep-corrected; SDI, Sleep Debt Index.STEP 1 included nighttime symptoms (AIS, SDI, and MSFsc), STEP 2 added daytime symptoms(JESS,BFI-J, and HADS), and STEP 3 additionaly adjusted for gender.∗p < 0.05.

Among older adults, subsyndromal insomnia, compared with normal sleepers, was significantly associated with higher AIS (OR = 3.43 [1.81–6.48]) and SDI scores (OR = 2.01 [1.08–3.74]; Table 6). Similar associations were observed in a comparison of normal sleepers with individuals with insomnia syndrome (AIS: OR = 5.38 [2.79–10.37]; SDI: OR = 2.75 [1.49–5.10]). In contrast, a comparison of subsyndromal insomnia with insomnia syndrome revealed that only BFI scores were significantly associated with insomnia syndrome (OR = 3.02 [1.30–7.03]).

4. Discussion

Comparisons of nocturnal sleep and daytime functioning between working-age and older adults revealed that working-age adults showed significantly worse scores across all symptom scales. This finding likely reflects the difficulty working-age adults face in obtaining adequate sleep while fulfilling multiple social roles such as work, household responsibilities, and childcare. Similarly, previous studies have indicated that occupational demands and family responsibilities are associated with poor sleep quality and an increased risk of insomnia [6,33]. These results further support the notion that sleep problems and daytime consequences are particularly pronounced in working-age adults. Additionally, the differences in nocturnal and daytime symptoms across insomnia types were examined in each age group. For both working-age and older adults, AIS and SDI scores worsened sequentially from normal sleepers to those with subsyndromal insomnia and to those with insomnia syndrome. This pattern suggests that insomnia severity and sleep debt increase with the progression of insomnia, regardless of age group.

In contrast, different patterns were observed for daytime fatigue and mental health scales. Among working-age adults, scores on the BFI and HADS were the highest in the insomnia syndrome group, followed by the subsyndromal insomnia group and normal sleepers. This finding indicates that, even in the subsyndromal stage, working-age adults may experience a certain degree of daytime fatigue and psychological distress. However, in older adults, no significant differences in fatigue or mental health scales were observed between normal sleepers and those with subsyndromal insomnia. These indicators worsened only in patients with insomnia syndrome.

Further, the findings suggests that among working-age adults, daytime functioning is impaired to some extent, even in subsyndromal insomnia without prominent daytime symptoms, whereas in older adults, daytime impairment becomes evident primarily when insomnia reaches a level that meets diagnostic criteria. Taken together, these results indicate qualitative differences between working-age and older adults in the manifestation of insomnia-related symptoms. In working-age adults, complaints of nocturnal symptoms alone may already be accompanied by daytime fatigue and psychological symptoms, highlighting the importance of early intervention even at the subsyndromal stage. In contrast, for older adults, the presence or absence of a formal insomnia diagnosis may be more clinically relevant when evaluating daytime dysfunction. Additionally, although both daytime sleepiness and fatigue were assessed in the present study, fatigue appeared to show greater discriminative ability than sleepiness in distinguishing insomnia types. Recently, sleep debt and insomnia have been reported to influence physical and psychological symptoms, with fatigue acting as a full mediator [34,35]. From a clinical perspective, placing greater emphasis on fatigue may be useful for assessing daytime dysfunction associated with insomnia.

Analysis of the factors associated with insomnia types further revealed age-specific patterns. Among working-age adults, subsyndromal insomnia, compared with normal sleep, was associated with an advanced sleep-wake rhythm, whereas insomnia syndrome was associated with sleep debt. This suggests that the nature of sleep problems differs depending on the presence of daytime dysfunction. Individuals reporting nocturnal symptoms alone may primarily experience difficulties related to sleep maintenance, including early morning awakening, whereas those with additional daytime dysfunction may experience insufficient sleep. That is, insomnia syndrome in working-age adults may be more appropriately conceptualized as a form of objective insomnia [36], although further investigations using objective sleep measures are warranted.

With respect to daytime functioning in working-age adults, the distinction between subsyndromal insomnia and insomnia syndrome was more strongly related to sleepiness and fatigue than to anxiety or depressive symptoms. This finding suggests that careful assessment of daytime sleepiness and fatigue is particularly important when working-age adults present with insomnia symptoms. When these symptoms are prominent, a more intensive insomnia treatment may be indicated.

Among older adults, both subsyndromal insomnia and insomnia syndrome were associated with sleep debt compared with normal sleepers. At first glance, this finding may suggest that objective insomnia is also common among older adults. However, the magnitude of sleep debt differed between age groups. In the present study, sleep debt among working-age adults with subsyndromal insomnia or insomnia syndrome ranged from approximately 1 h 5 min to 1 h 26 min, whereas in older adults, it ranged from approximately 55 min to 1 h 10 min. Thus, sleep debt among older adults was relatively low. This interpretation is further supported by the finding that daytime functioning did not differ significantly between normal sleepers and those with subsyndromal insomnia or insomnia syndrome in older adults in this study. Perceived sleep loss may contribute to a longer bedtime, consistent with the findings of Yoshiike et al. [20]. Nevertheless, previous studies have shown that sleep debt exceeding 1 h is associated with impaired daytime functioning [27]. Therefore, the idea that a subset of older adults with insomnia syndrome may experience objective insomnia cannot be ruled out. This distinction may have implications for treatment selection in older adults, particularly when considering pharmacological versus non-pharmacological interventions, such as cognitive behavioral therapy for insomnia (CBT-I) [34].

Additionally, among older adults, fatigue, rather than sleepiness, emerged as a key factor in distinguishing insomnia syndrome from subsyndromal insomnia. In general, aging is associated with reductions in daytime sleep propensity, nocturnal sleep continuity, and slow-wave sleep [37]. Furthermore, diminished restorative processes related to slow-wave sleep are linked to impaired physical and cognitive recovery [38]. These changes, together with reduced sleep propensity, may contribute to a dissociation in which older adults experience fatigue more prominently than sleepiness. These findings suggests that fatigue may represent a central feature of daytime dysfunction in late-life insomnia. Clinically, careful assessment of fatigue may be particularly informative when evaluating insomnia in older adults. From an epidemiological perspective, incorporating fatigue measures may also improve the characterization of insomnia-related problems in this population.

In summary, the present findings indicate that working-age and older adults differ in terms of nocturnal and daytime symptoms. Insomnia type was associated with different factors in each age group: among working-age adults, subsyndromal insomnia was related to sleep–wake rhythm characteristics, whereas insomnia syndrome was more strongly associated with sleep debt, sleepiness, and fatigue; among older adults, insomnia syndrome was primarily associated with sleep debt and fatigue, although the magnitude of sleep debt was smaller than that observed in working-age adults. These findings suggest that objective insomnia may be more prevalent among working-age adults, whereas subjective insomnia may be more common among older adults. Consequently, pharmacological treatments may play a larger role in working-age adults, while CBT-I may be particularly suitable for older adults [34]. Further research is needed to clarify these distinctions, particularly by examining insomnia subtypes using both objective and subjective sleep parameters and by developing age-specific treatment strategies.

5. Limitations

This present study has some limitations. First, the proportion of male participants was relatively high. Previous epidemiological studies have reported a higher prevalence of insomnia in women [2]. Although sex was included as a covariate in the logistic regression analyses, the imbalanced sex distribution may introduce sampling bias and limit the generalizability of the findings. Future studies should further examine the potential sex differences in insomnia-related symptoms and daytime dysfunction. Second, a part of daytime dysfunction in the patients’ complaints was assessed in the present study. According to the ICSD-3, daytime consequences of insomnia may include not only fatigue and sleepiness, but also impairments in attention, concentration, and memory, as well as behavioral problems such as hyperactivity and impulsivity [25]. Future research should incorporate a broader range of daytime functioning measures, including cognitive performance, to provide a more comprehensive understanding of insomnia-related impairments. Additionally, further studies are needed to identify intervention strategies tailored to the specific sleep characteristics and clinical needs of different age groups. Finally, longitudinal and interventional designs are particularly valuable for clarifying causal relationships and optimizing age-specific treatment approaches.

6. Conclusion

The present study demonstrated that even when insomnia was present, the degree and pattern of daytime dysfunction differed between working-age and older adults. Furthermore, different insomnia types were associated with distinct symptom profiles in each age group. These findings underscore the importance of age-specific assessment and intervention strategies for insomnia and highlight the potential value of focusing on fatigue when evaluating the daytime impact of sleep problems.

CRediT authorship contribution statement

Koyo Higami: Writing – original draft, Formal analysis. Masumi Osao: Writing – review & editing, Conceptualization. Isa Okajima: Writing – review & editing, Supervision, Investigation, Funding acquisition, Conceptualization.

Data availability statement

Data supporting the findings of this study are available from the corresponding author upon request.

Declaration of generative AI and AI-assisted technologies in the manuscript preparation process

None.

Funding

This work was supported by the Japan Society for the Promotion of Science [JSPS KAKENHIGrant Number, 19K03348].

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Isa Okajima reports financial support was provided by Japan Society for the Promotion of Science. The authors declare the following financial interests and/or personal relationships that may be considered as potential competing interests: Isa Okajima reports relationships with NEC Solution Innovators Ltd. (consulting or advisory roles and research funding), Infocom Corporation (research funding), Otsuka Pharmaceutical Co., Ltd. (speaking and lecture fees), MSD K.K. (speaking and lecture fees), Eisai Co., Ltd. (speaking and lecture fees), Daiichi Sankyo Co., Ltd. (speaking and lecture fees), and Suntory Wellness Ltd. (consulting andor advisory roles). Masumi Osao belongs to the Department of Somnology at, Tokyo Medical University. The Department of Somnology, Tokyo Medical University, is an endowment department supported by grants from Philips Japan Co., Ltd. and Koike Medical Co., Ltd. All other authors declare that they have no known competing financial interests or personal relationships that could have appeared to influenced the work reported in this study paper. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.