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. Author manuscript; available in PMC: 2026 Jun 3.
Published in final edited form as: J Sleep Res. 2025 Jun 3;35(1):e70062. doi: 10.1111/jsr.70062

Evening chronotype is associated with daytime impairment and differential sleep and circadian response to a sleep extension manipulation in short sleeping adolescents

Stacey L Simon 1,2, Emily Cooper 2, Angel Bernard 2, Anne E Bowen 1,2, Sydney Holtman 2,3, John T Brinton 2, Stephen MM Hawkins 1,2, Melanie G Cree 1,2, Kristen J Nadeau 1,2, Kenneth P Wright Jr 1,3
PMCID: PMC12353921  NIHMSID: NIHMS2070587  PMID: 40458955

Abstract

Adolescents frequently experience insufficient sleep which can negatively impact physical and mental health. We sought to examine differences in symptoms of insufficient sleep and response to a sleep extension manipulation by chronotype in habitually short-sleeping adolescents. Twenty-six adolescents with insufficient sleep (≤7 hours on school nights) were randomized to one week of typical sleep (TS; usual school schedule) and sleep extension (EXT; ≥1-hour additional time in bed) in counterbalanced order. Questionnaires (Morningness/Eveningness Scale for Children, PROMIS Pediatric Anxiety and Depression Scales, Chronic Sleep Reduction Questionnaire) were completed at screening, and actigraphy-estimated sleep and dim-light salivary melatonin onset and offset (DLMOn/Off) were assessed during both conditions. A slight majority of participants identified as evening types (57.7%). Evening types had significantly greater depression and anxiety symptoms, more chronic sleep restriction, and more daytime sequelae of insufficient sleep (irritability, loss of energy, sleepiness) compared to morning types. Both morning/intermediate and evening types significantly advanced sleep onset time and increased sleep duration from TS to EXT (all p<0.001). However, only evening types significantly delayed sleep offset from TS to EXT (p=0.049) while morning/intermediate types did not change timing of sleep offset between conditions (p>0.05). The phase angle between DLMOn and sleep onset narrowed significantly only for morning/intermediate types (p=0.02), while the phase angle between DLMOn and sleep offset widened significantly only for evening types (p=0.021). To improve health, individualized approaches to improving sleep health should be considered for habitually short-sleeping adolescents based on chronotype.

Keywords: dim light salivary melatonin, melatonin onset, phase angle, actigraphy, pediatrics, mood

Introduction

A confluence of factors result in pervasive chronic insufficient sleep among United States adolescents, with nearly 80% of high-school students obtaining less than the recommended 8–10h of sleep per night1,2. Adolescence is characterized by a puberty-related physiological delay in circadian rhythms and slowed buildup of homeostatic sleep drive that, together with high academic and psychosocial demands, results in late bedtimes3. Imposed early school start times mean that many adolescents are required to wake early in the morning on school days, during their biological night when melatonin levels are high3. Insufficient sleep is associated with significant daytime impairment in cognitive, academic, psychosocial, and physiological domains3–5. Therefore, strategies to improve adolescent sleep health by increasing sleep duration are urgently needed.

Chronotype, the temporal organization of an individual’s behavior, also changes during adolescence with an increase in eveningness compared to childhood6,7. Differential effects of chronotype on sleep health have been previously reported such that adolescents with evening chronotype report later bed and wake times on non-school nights, more social jetlag, and greater insomnia symptoms compared to morning or intermediate types8–10. Evening chronotype may also place adolescents at increased risk for daytime dysfunction. Adolescents with eveningness had impaired executive function, poorer academic performance, and endorsed engaging in risky behaviors such as drowsy driving and alcohol consumption compared to morning/intermediate types11–13. Evening type adolescents also demonstrate more emotional and behavioral problems including greater depression and anxiety symptoms, aggression, and suicidal thoughts than other chronotypes9,10,14,15.

There has been limited research exploring differences in response to sleep interventions by chronotype. A randomized-control trial comparing a 4-session behavioral intervention aimed at increasing sleep by 1–1.5h/night in 8–11-year-old children who obtain <9.5h sleep/night found no difference in treatment response by chronotype16. A study of adults with insomnia who completed group cognitive behavioral therapy for insomnia (CBT-I) in an outpatient sleep disorders center found that all chronotypes had improvements in insomnia following the intervention, but those with eveningness had less improvement in depressive symptoms compared to morning types17. However, adolescent responses to sleep intervention by chronotype have not been examined, despite adolescence being a high-risk period for both greater eveningness and chronic insufficient sleep.

Given the preponderance of eveningness in adolescence combined with increased risk for poor sleep, health, and behavior outcomes among evening type adolescents, chronotype is an important factor to examine in this developmental stage. The goal of the current study was to examine differences in daytime symptoms of insufficient sleep and change in sleep and circadian rhythms in response to a sleep extension manipulation by chronotype in a sample of habitually short-sleeping adolescents.

Methods

Participants

Individuals attending high school ages of 14–19 years with habitually insufficient sleep (≤7 hours of sleep per night on school nights) were recruited via advertisements in high school newsletters, flyers at area pediatrician practices, and social media postings (e.g., Facebook, Instagram). Study enrollment occurred October 2018-April 2021; research activities were paused from March to November 2020 due to COVID-19 shutdowns. Inclusion criteria included body mass index (BMI) in the 5th-95th percentile for age and sex, habitually physically inactive (reported < 3h of regular physical activity per week), and late pubertal stage (Tanner stage IV or V). Exclusion criteria were use of medications that affect sleep (i.e., stimulants, atypical antipsychotics, current use of oral steroids), regular use of melatonin or other sleep aids, a prior reported diagnosis of a sleep disorder (i.e., insomnia, delayed sleep-wake phase disorder, obstructive sleep apnea), or an IQ<70 or severe mental illness that may impact sleep or ability to complete study measures. Online or home schooling, or schedules that prohibited completion of the sleep manipulation (e.g., night shift employment) were exclusionary. The Colorado Multiple Institutional Review Board and Scientific Advisory and Review Board approved the protocol. All participants ≥18 years or the parent/guardian of participants <18 years signed an informed consent, and participants <18 years provided assent prior to participation in any study procedures. The study was registered on Clinicaltrials.gov (NCT03500458) and full methodology has been reported previously18.

Procedures

Participants who expressed interest in the study completed an online screening survey querying inclusion/exclusion criterion, which was then verified via phone. Following informed consent, participants completed one week of home monitoring with actigraphy to verify baseline sleep duration. An in-person screening visit with a physical exam confirmed medical history and Tanner staging. Anthropometrics were obtained via Bod Pod® assessment, height was obtained with a stadiometer, and questionnaires were completed.

The study utilized a randomized cross-over design (see Figure 1). Participants completed both one week of typical sleep (TS) and one week of sleep extension (EXT) in random, counterbalanced order with a one-month washout period in between conditions. Participants were asked to refrain from daytime napping during both conditions. Sleep schedules were maintained in the home environment for five nights (Sunday-Thursday), monitored with actigraphy and concurrent sleep diary. Following each sleep week, participants were admitted to the Children’s Hospital Colorado Clinical and Translational Research Center for identical in-laboratory overnight study visits beginning at 16:00 on Thursday. Participants remained in dim-light conditions (<10 lux maximum in the room and ~2 lux in the angle of gaze measured with a light meter) maintained with blackout curtains and light-blocking goggles (shade 5 ultraviolet and infrared blocking) for the duration of the study visit. Hourly salivary melatonin samples were obtained in the evening from 17:00 until prescribed bedtime for that sleep condition, and again the following morning. Participants maintained their prescribed sleep schedules during the overnight study visit. All study procedures took place during the academic year, avoiding daylight savings time changes, major holidays, school breaks, and travel across two or more time zones. Participants were compensated up to a total of $220 for their participation in the study.

Figure 1.

Figure 1.

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Randomized crossover trial design. Following screening and informed consent, participants were randomized to complete either one week of Typical Sleep (TS, usual school sleep schedule) or one week of Sleep Extension (EXT, ≥1h increased time in bed) first, completing the other condition subsequently.

Sleep Conditions

Typical Sleep (TS):

Participants were asked to follow their usual school-week sleep schedule, consistent with their screening week home monitoring.

Sleep Extension (EXT):

Through collaborative conversation with participants, parents, and study staff, participants were assigned an individualized sleep schedule that allowed them to obtain at least one-hour additional time in bed compared to TS. Behavioral recommendations to promote healthy sleep were provided to facilitate adherence, and anticipated barriers were discussed. Behavioral recommendations included limiting evening electronic use, adding a brief bedtime routine, scheduling homework to be completed earlier in the day, and increasing parental support14.

Measures

Chronotype:

The Morningness/ Eveningness Scale for Children (MESC)19 was completed at screening to assess sleep timing preference. The measure has demonstrated full-scale reliability in youth20 and includes 10 questions about preferred timing of bedtime, waketime, and daily activities (i.e. taking a test or attending gym class). Scores range from 10–42, with lower scores indicating greater eveningness and higher scores indicating a greater morning preference. Using cut-off points based on the 25th to 75th percentiles, individuals with scores of 10–23 were categorized as “evening-type”, 24–27 as “intermediate-type”, and 28–40 were categorized as “morning-type”21. For the current analyses, evening types were compared to morning and intermediate types.

Anxiety & Depressive Symptoms:

The Patient-Reported Outcomes Measurement Information System (PROMIS) Pediatric Anxiety and Depression v2.0 short forms were completed at screening and assessed mood symptoms over the past week19. The 8-item questionnaires instruct participants to rate the frequency of their mood symptoms on a 5-point Likert-type scale ranging from Never (1) to Almost Always (5). Raw scores are converted to T-scores using the Health Measures Scoring Service20; T-scores below 55 are considered “normal,” 55–60 are “mild”, 60–70 are “moderate,” and >70 considered “severe.”21

Symptoms of Insufficient Sleep:

The Chronic Sleep Reduction Questionnaire (CSRQ22) is a 20-item measure that evaluates symptoms of insufficient sleep over the past two weeks and was completed by participants at the screening visit. Items are summed to obtain a Total Score and four subscales: Shortness of Sleep, Loss of Energy, Irritability, and Sleepiness. Higher scores indicate more chronic sleep reduction.

Sleep/Wake:

The Actiwatch Spectrum Plus (Philips Respironics, Bend, OR) actigraphy watch was worn continuously on the non-dominant wrist to estimate sleep during both conditions. A concurrent daily online sleep diary was completed to confirm adherence to the sleep schedule in real time and facilitate accurate scoring of the actigraphy data. Actiware Sleep v6 software (Philips Respironics, Pittsburgh, PA) and standard scoring rules were used to calculate sleep/wake variables, including weekday sleep onset and offset, and total sleep duration, as well as minutes of wake after sleep onset, sleep onset latency, and sleep efficiency15.

Light exposure:

Red, green, and blue colored light sensors on the Actiwatch Spectrum Plus (Philips Respironics, Bend, OR) were integrated to determine white light illuminance (lux) per minute of wear. Data were calculated as average lux values over 24 hours.

Circadian Rhythms:

During the overnight study visit, participants provided hourly saliva samples (~2 mL) during waking hours from 17:00 until scheduled bedtime and from 07:00 until 12:00 the following morning. Brushing teeth and eating, drinking or standing within 15 minutes of each saliva sample were prohibited to avoid alterations melatonin levels16 or sample contamination. Salivary samples were assayed for melatonin using double-antibody radioimmunoassay, based on the G280 antimelatonin antibody, validated by reverse-phase column extraction (Alpco Laboratories, Salem, NH; 1.00 pg/mL sensitivity, 8.10% Inter-assay coefficient of variation)17 in the University of Colorado Anschutz CTRC Core Laboratory. Using linear interpolation, dim light melatonin onset (DLMOn) was calculated as the clock time that melatonin levels rose and remained above 4 pg/mL and dim light melatonin offset (DLMOff) as the clock time that melatonin levels dropped and remained below 4 pg/mL. Phase relationships were calculated as the difference in hours between DLMOn or DLMOff and actigraphy-assessed sleep onset and offset times. Duration of melatonin secretion was calculated as hours from DLMOn to DLMOff.

Statistical Analysis

Descriptive statistics are reported as mean ± standard deviation in tables and figures. Differences in demographic and screening questionnaire scores by chronotype were examined with independent-samples t-tests, chi-squared, or Fisher’s exact tests, as appropriate. Linear mixed models were fit to predict sleep and circadian variables based on sleep condition (TS vs EXT), chronotype (morning/intermediate vs evening), and an interaction between sleep condition and chronotype. Randomization sequence (condition order) and condition were included as covariates. Models were adjusted for season of study participation based on a likelihood ratio test with a p-value <0.05 indicating that the covariate contributed significantly to the model and should be included. Model-estimated means and 95% confidence intervals (95% CI) for each chronotype at each sleep condition and p-values from Tukey’s honestly significant difference (HSD) for pairwise comparisons of the model-estimated means are reported. All analyses were completed with R studio (R version 4.3.3, R study Team). For all statistical tests, alpha = 0.05 was set as the level for indicating statistical significance.

Results

The sample included 26 participants, age 16.1 ±1.2 years, majority female and non-Hispanic White; see Table 1 for all participant characteristics. A slight majority of participants identified as evening types (57.7%). As previously reported, during EXT compared to TS, sleep duration for the entire sample increased by 1.4±0.7 hours (p<0.001) and bedtimes advanced by 1.4±0.9 hours (p<0.001), while neither waketime nor DLMOn/Off significantly changed (all p>0.05)18.

Table 1.

Participant Characteristics

Variable All
(N = 26)		 Morning/Intermediate
(N=11)		 Evening
(N=15)
Age (years) 16.1 ± 1.2 16.0 ± 1.4 16.1 ± 1.0
BMI percentile for age and sex 54.6 ± 23.6 53.7 ± 25.9 55.2 ± 22.7
Tanner Stage V 26 (100%)
Sex, N (%)
Male 8 (30.8%) 5 (45.5%) 3 (20.0%)
Female 18 (69.2%) 6 (54.5%) 12 (80.0%)
Race, N (%)
American Indian/Alaska Native 1 (3.8%) 0 (0%) 1 (6.7%)
Asian 4 (15.4%) 1 (9.1%) 3 (20.0%)
White 21 (80.8%) 10 (90.9%) 11 (73.3%)
Ethnicity, N (%)
Hispanic 5 (19.2%) 3 (27.3%) 2 (13.3%)
Not Hispanic 21 (80.8%) 8 (72.7%) 13 (86.7%)
Questionnaires
Morningness Eveningness Scale for Childrena 23.3 ± 5.8 28.8 ± 3.8 19.3 ± 3.0*
PROMIS Depressionb 52.1 ±10.9 42.9 ± 6.9 58.9 ± 7.9*
PROMIS Anxietyc 50.7 ± 10.1 44.3 ± 7.1 55.6 ± 9.5*
Chronic Sleep Reduction Questionnaired
Total Score 40.0 ± 9.9 33.1 ± 10.9 45.1 ±5.2*
Shortness of Sleep 13.8 ±2.8 12.0 ± 3.4 15.1 ± 1.3*
Loss of Energy 9.5 ± 2.2 7.6 ± 1.1 10.8 ± 1.9*
Irritability 8.5 ± 2.7 7.0 ± 2.1 9.7 ± 2.6^
Sleepiness 8.9 ± 1.6 8.1 ± 1.4 9.5 ± 1.6^
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Data are reported as mean ± standard deviation or N (%).

a

Range = 10–42, higher scores indicate greater morningness.

b

Range = 0–100, higher scores indicate greater depressive symptoms.

c

Range =0–100, higher scores indicate greater anxiety symptoms.

d

Range = 20–60, higher scores indicate greater symptoms of sleep reduction.

*

p<0.01,

^

p<0.05 from independent samples t-tests comparing Morning/Intermediate and Evening types.

Differential Daytime Characteristics by Chronotype

At screening, evening types had significantly greater depression (p<0.001) and anxiety symptoms (p=0.003) compared to morning types. Evening types had a greater degree of chronic sleep restriction (CSRQ total; p=0.001), as well as more daytime sequelae of insufficient sleep including irritability (p=0.01), loss of energy (p<0.001), and sleepiness (p=0.03) than morning types. There was no difference in age by chronotype (p=0.80; see Table 1).

Differences in Sleep & Circadian Variables at TS and EXT by Chronotype

There were no significant differences in any sleep or circadian variables or average lux between morning/intermediate and evening types at TS. At EXT, evening types showed on average an ~1.7h significantly later DLMOff (p=0.008) and a ~0.7h significantly later sleep midpoint compared to morning types (p=0.039; see Figure 2, Table 2, and Table 3). Further, evening types had a significantly later sleep offset time by 0.6h at EXT after controlling for season of study participation (p=0.034; see Figure 2 and Table 2).

Figure 2.

Figure 2.

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Actigraphy-estimated sleep and melatonin variables at Typical Sleep (TS) and Sleep Extension (EXT) for morning/intermediate types (top) and evening types (bottom). At EXT, evening types had significantly later dim light melatonin offset (DLMOff, depicted by open circles; evening=10: 09 [95% CI: 09:21, 10:57], morning/intermediate=08:28 [95% CI: 07:38, 09:17]; p=0.008), and sleep midpoint (depicted by diamond; evening = 03:31 [95% CI: 03:07, 03:54], morning/intermediate=02:50 [95% CI: 02:22, 03:18]; p=0.039), and sleep offset (controlling for season of study participation; depicted by upward triangle; evening = 07:23 [95% CI: 07:02, 07:45], morning/intermediate = 06:42 [95% CI: 06:20, 07:13]; p = 0.03) compared to morning types. Only evening types significantly delayed sleep offset time from TS to EXT (TS=06:54 [95% CI: 06:34, 07:16], EXT=07:23 [95% CI: 07:02, 07:45]; p=0.049) while morning/intermediate types did not change sleep offset times between conditions (p>0.05). Change in sleep midpoint from TS to EXT was significant only for morning/intermediate types (TS=03:34 [95% CI: 03:05, 04:01], EXT=02:50 [95% CI: 02:22, 03:18]; p=0.005) but not evening types (p>0.05). The phase angle between dim light melatonin onset (DLMOn) and sleep onset narrowed significantly only for morning/intermediate types (TS=3.9h [95% CI: 3.0, 4.8], EXT=2.7h [95% CI: 1.8, 3.6]; p=0.02), while the phase angle between DLMOn and sleep offset widened significantly only for evening types (TS=10h [95% CI: 9.1, 10.8], EXT=11.2h [95% CI: 10.4, 12.1]; p=0.02).

Table 2.

Average Actigraphy-Assessed Sleep Variables at Typical Sleep and Sleep Extension by Chronotype

P-values
Model-Estimated Means (95% CI) Comparison by Sleep Condition Comparison by Chronotype
Outcome Chronotype Typical Sleep Sleep Extension Morning/ Intermediate Evening Typical Sleep Sleep Extension
Sleep Onset (clock time) Morning/ Intermediate 00:26 (23:47, 01:04) 22:52 (22:14, 23:31) <0.001 <0.001 0.346 0.064
Evening 00:50 (00:18, 01:23) 23:41 (23:09. 00:14)
Sleep Offset* (clock time) Morning/ Intermediate 06:39 (06:12, 07:06) 06:42 (06:20, 07: 06) 0.635 0.049 0.358 0.030
Evening 06:54 (06:34, 07:16) 07:23 (07:02, 07:45)
Sleep midpoint (clock time) Morning/ Intermediate 03:34 (03:05, 04:01) 02:50 (02:22, 03:18) 0.005 0.111 0.385 0.039
Evening 03:50 (03:26, 04:14) 03:31 (03:07, 03:54)
Sleep duration (hours) Morning/ Intermediate 5.76 (5.26, 6.26) 7.15 (6.65, 7.65) <0.001 <0.001 0.359 0.322
Evening 5.45 (5.03, 5.87) 6.81 (6.39, 7.24)
Sleep efficiency (%) Morning/ Intermediate 88.65 (86.37, 90.94) 87.11 (84.83, 89.4) 0.221 0.159 0.279 0.289
Evening 86.97 (85.04, 88.9) 85.47 (83.54, 87.39)
Sleep onset latency (min) Morning/ Intermediate 5.94 (2.7, 9.18) 9.77 (6.53, 13.01) 0.093 0.006 0.933 0.476
Evening 5.76 (3.02, 8.49) 11.33 (8.6, 14.06)
Wake after sleep onset (min) Morning/ Intermediate 29.27 (19.61, 38.93) 46.99 (37.33, 56.65) 0.005 0.001 0.702 0.737
Evening 31.76 (23.61, 39.92) 49.19 (41.03, 57.34)
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*

Model adjusted for season of study participation.

Table 3.

Average Circadian Variables at Typical Sleep and Sleep Extension by Chronotype

P-values
Model-Estimated Means (95% CI) Comparison by Sleep Condition Comparison by Chronotype
Outcome Chronotype Typical Sleep Sleep Extension Morning/ Intermediate Evening Typical Sleep Sleep Extension
Dim light melatonin onset (DLMOn; clock time) Morning/ Intermediate 20:33 (19:34, 21:32) 20:13 (19:16, 21:10) 0.497 0.112 0.539 0.969
Evening 20:58 (20:08, 21:47) 20:14 (19:23, 21:05)
Dim light melatonin offset (DLMOff; clock time) Morning/ Intermediate 09:02 (08:06, 09:59) 08:28 (07:38, 09:17) 0.248 0.727 0.120 0.008
Evening 10:00 (09:17, 10:43) 10:09 (09:21, 10:57)
Phase angle between DLMOn and sleep onset (hours) Morning/ Intermediate 3.9 (2.98, 4.83) 2.67 (1.78, 3.57) 0.020 0.504 0.938 0.154
Evening 3.86 (3.08, 4.63) 3.56 (2.76, 4.36)
Phase angle between DLMOn and sleep offset (hours) Morning/ Intermediate 10.14 (9.15, 11.13) 10.61 (9.66, 11.57) 0.427 0.021 0.771 0.351
Evening 9.95 (9.12, 10.78) 11.23 (10.37, 12.09)
Phase angle between DLMOff and sleep offset (hours) Morning/ Intermediate 2.41 (1.27, 3.56) 1.61 (0.59, 2.63) 0.154 0.950 0.418 0.066
Evening 3.02 (2.15, 3.89) 2.99 (2.02, 3.96)
Duration of melatonin Secretion (hours) Morning/ Intermediate 12.62 (10.98, 14.26) 12.31 (10.99, 13.63) 0.732 0.379 0.764 0.186
Evening 12.93 (11.79, 14.07) 13.69 (12.19, 15.19)
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Change in Sleep & Circadian Variables from TS to EXT by Chronotype

Figure 2 depicts change in sleep and circadian variables from TS to EXT for morning/intermediate and evening types. Both morning/intermediate and evening types significantly advanced sleep onset times and increased sleep duration from TS to EXT (all p<0.001). However, only evening types significantly delayed sleep offset from TS to EXT by ~0.5h (p=0.049) while morning/intermediate types did not change sleep offset between conditions (p>0.10), while controlling for season of study participation. Change in sleep midpoint from TS to EXT was significantly earlier by ~0.7h for morning/intermediate types (p=0.005) but not for evening types (p>0.10). WASO significantly increased for both groups from TS to EXT (both p<0.01), while sleep onset latency significantly increased only for the evening types by 5.5min (p=0.006; see Table 2). The phase angle between DLMOn and sleep onset narrowed significantly by ~1.2h for morning/intermediate types (p=0.02), while the phase angle between DLMOn and sleep offset widened significantly by ~1.2h for evening types (p=0.021; see Figure 2 and Table 3). There were no significant changes in melatonin duration or average lux from TS to EXT for either chronotype (p-values>0.10).

Conclusions

In this sample of habitually short-sleeping adolescents studied during the academic year, evening types exhibited more baseline impairment including greater anxiety and depression symptoms, irritability, and daytime sleepiness, than morning/intermediate types. No differences in sleep were observed by chronotype during TS, but evening types had later sleep midpoints and later DLMOff at EXT. While both chronotypes increased sleep duration and advanced sleep onset, only evening types delayed sleep offset from TS to EXT. Sleep midpoint shifted earlier and the phase angle between DLMOn and sleep onset narrowed from TS to EXT for morning/intermediate types only, while the phase angle between DLMOn and sleep offset widened from TS to EXT only for evening types.

The finding that sleep did not differ by chronotype during typical sleep is contrary to prior research showing that adolescents with eveningness have later bed and wake times9,10. However, this may be due to the inclusion criteria that required all participants to have insufficient sleep on school nights. Notably, evening types had later sleep midpoints and later DLMOff during EXT compared to morning/intermediate types despite no difference in total sleep duration at EXT by chronotype. Both chronotypes significantly advanced sleep onset, but only evening types also delayed sleep offset from TS to EXT. This suggests that while morning/intermediate types relied on earlier bedtimes to achieve additional sleep, evening types additionally attempted to extend sleep by delaying wake. This is consistent with a study of adults with eveningness who reported increasing sleep duration on weekends by delaying wake times, in contrast to morning types23.

However, despite advanced sleep onset times, we did not see a significant difference in DLMOn by chronotype at EXT. Crowley et al. found that having a later baseline circadian phase was associated with greater DLMOn phase advancement following bedtime advancement and morning bright light therapy25. These authors posited that individuals with earlier circadian phase may not require a change in DLMOn to advance bedtimes and increase sleep duration25. It may be that individuals with earlier baseline DLMOn push past the homeostatic drive for sleep to engage in other activities such as homework or electronics, delaying bedtimes and restricting sleep duration. When instructed to stop activities and go to bed earlier, they may be more easily able to advance bedtimes. In contrast, individuals with later initial circadian phases may not be physiologically sleepy at earlier bedtimes and may benefit more from later waketimes and/or circadian rhythm interventions to advance DLMOn. Future research should assess chronotype and tailor sleep and circadian interventions appropriately for optimal sleep and circadian outcomes.

Evening type participants in the current study endorsed more symptoms of anxiety, depression, irritability, and daytime sleepiness. This is consistent with existing literature that has found evening type adolescents have more emotional and behavioral problems compared to morning types9,10,14,15. While not possible to assess in the current sample given the BMI-related inclusion criteria, prior research has been mixed with regards to the association between chronotype and physical health outcomes. Some studies have found that adolescents and young adults with evening chronotype have higher BMI z-score, less healthy dietary behaviors, and greater cardiometabolic health risk24,25. Conversely, two studies found no association between chronotype with physical health outcomes in adolescents and university students26,27. Thus, future research examining the impact of chronotype on physical health in adolescents is warranted.

Objective measurement of sleep behavior in the home environment coupled with objective melatonin sampling to determine circadian timing is a significant strength of the study. However, there were also notable limitations. The sample size was relatively small and homogenous; future research with larger and more diverse samples is recommended. The one week sleep manipulation and monitoring period were relatively brief, and it may be beneficial to evaluate differences by chronotype in response to longer-term sleep interventions.

The findings of the current study suggest that individualized approaches to improving sleep health should be considered for habitually short-sleeping adolescents based on chronotype. Notably, this contrasts with prior studies that found no difference in response to sleep intervention by chronotype in other age groups including younger children and adults16,17. Because adolescence is a unique developmental period for sleep characterized by physiological and psychosocial factors that increase risk for insufficient duration and late timing of sleep, it may be important to consider chronotype in future sleep and circadian interventions in this developmental stage. As delaying wake is less feasible with fixed and traditionally early school start times, particular consideration is needed for evening chronotypes, such as considering later school start times28. Further research is needed to examine the impact of improved sleep and circadian health on adolescent health and wellbeing in all chronotypes.

Acknowledgements:

Data is available upon reasonable request to the corresponding author. Funding was provided by the National Institute for Health (NIH NIDDK K23DK117021; NIH BIRCWH K12HD057022; and NIH NCATS Colorado CTSA UL1 TR002535). KPW has served as a paid or unpaid member of scientific advisory boards for Circadian Therapeutics, LTD, Circadian Biotherapies, Inc., and the U.S. Army Medical Research and Materiel Command–Walter Reed Army Institute of Research and has received research support/donated materials from DuPont Nutrition & Biosciences, Grain Processing Corporation, and Friesland Campina Innovation Centre; MGC has served as a consultant for Pollie Inc and received donated materials from Abbott; SLS received donated research materials from Nova Biomedical. No other authors report disclosures or conflicts of interest. This study was approved by the Colorado Multiple Institutional Review Board and written informed consent/assent was obtained from parents/guardians and participants prior to any study procedures. Clinical trials registration: NCT03500458.

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