0125 Sleep Deprivation Impairs the Ability to Overcome Pre-Existing Framing Bias

Introduction

When presented with a choice between sure gains or losses versus gambles, people tend to select sure gains over gambles, but gambles over sure losses. This pre-existing framing bias is embedded in the Framed Gambling Task (FGT), in which subjects choose between a sure option (gain or loss) and a gamble (card from one of two decks). For optimal performance, subjects need to recognize that one deck (‘good deck’) results in better average outcomes than the other deck (‘bad deck’) and select the gamble or sure option depending on the deck (good/bad) rather than the frame (sure loss/gain). A speeded version of the FGT, with 2s response deadlines to induce time pressure, was used in a laboratory total sleep deprivation (TSD) study to determine the impact of sleep loss on the ability to overcome pre-existing framing bias.

Methods

Eight-six subjects (ages 21–38; 47 females) were randomized (2:1 ratio) to a TSD condition (n=56) or control condition (n=30). They completed the speeded FGT at 11:00 on the baseline day (session 1), and again the following day (session 2) after 27h of wakefulness (TSD group) or 3h of wakefulness (control group). Performance accuracy was defined in terms of optimal task performance, i.e., gambling when the good deck was presented and not gambling when the bad deck was presented. Each test bout had 72 trials across three trial blocks.

Results

Accuracy improved across trial blocks (F1,84=20.44, p<0.001). In session 2, the TSD group showed lower accuracy (condition by session interaction: F2,84=4.15, p=0.045) and less improvement across trial blocks (condition by session by trial block interaction: F2,168=3.97, p=0.021) than the control group. Even under TSD, the frequency of response timeouts (RT>2s) was low (<3.5% of trials).

Conclusion

Sleep deprivation degraded FGT performance under time pressure, indicating reduced ability to overcome pre-existing framing bias.

Support

PRMRP award W81XWH-16-1-0319

0297 Relationship Between Sleepiness Symptoms Questionnaire Ratings and Psychomotor Vigilance Performance in a Total Sleep Deprivation Study

Introduction

While measuring the neurobehavioral consequences of sleepiness is arguably best done with performance tasks providing objective assessments of impairment, this often proves challenging in real-world operational settings. Self-report instruments measuring subjective sleepiness provide a practical alternative, but field studies generally fail to show high correlation between objective and subjective assessments of impairment. The Sleepiness Symptoms Questionnaire (SSQ) is a self-report instrument developed to address this issue by asking about observable symptoms of sleepiness and (motor vehicle driving) performance impairment. In a laboratory sleep deprivation study, we compared SSQ sleepiness ratings to performance impairment on a 10min psychomotor vigilance test (PVT).

Methods

N=12 healthy normal sleepers (ages 21-39y, 6 females) participated in a 4-day in-laboratory study. After a baseline day (9h time in bed; 22:00-07:00), subjects underwent 38h total sleep deprivation (TSD) followed by a recovery day (9h time in bed; 22:00-07:00). As part of neurobehavioral testing throughout the experiment, subjects completed the SSQ and PVT back to back at 6.5h, 14.5h, 22.5h, and 30.5h TSD, and 6.5h after recovery sleep. Data from one subject were incomplete and discarded prior to analysis.

Results

As TSD progressed, the SSQ sleepiness ratings and the number of lapses (RTs>500ms) on the PVT were elevated, with sleepiness and performance impairment peaking at 22.5h TSD. Both measures returned to baseline levels after recovery sleep. Mixed-effects analysis of covariance showed moderate correlation between SSQ ratings and PVT lapses (r=0.44, F1,43=24.1, p<0.001).

Conclusion

Self-reported sleepiness on the SSQ reflected the expected homeostatic and circadian changes in sleep pressure during TSD and following recovery sleep, as did PVT performance impairment. The moderate correlation between subjective ratings on the SSQ, with its emphasis on observable sleepiness symptoms, and objective impairment on the PVT suggests that the SSQ may be a reasonably reliable instrument for measuring sleepiness under conditions of acute sleep deprivation.

Support

Jazz Pharmaceuticals

0301 Different Indices of Vigilant Attention During Sleep Deprivation: Evidence of Multiple Vigilance Constructs?

Introduction

Total sleep deprivation (TSD) causes profound vigilant attention deficits, with large, trait-like individual differences, as evidenced convincingly by response lapses on the psychomotor vigilance test (PVT). There is debate, however, about the role of vigilant attention deficits in the effects of TSD on other speeded performance tasks besides the PVT. We addressed this issue by testing whether PVT response lapses are related to delays in responding to stimuli under strict deadlines in two decision making tasks.

Methods

N=54 healthy adults (aged 21-38y; 31 females) completed an in-laboratory TSD study. Following a 10h baseline sleep opportunity, cognitive testing occurred after 25h and 29h of TSD (09:00 and 13:00). Testing included an AX continuous performance task with switch (AX-CPTs), which is a dynamic decision making task requiring subjects to respond to a frequently occurring cue-probe combination; an identical pairs continuous performance task (CPT-IP), which is a 1-back go/no-go task; and a 10min PVT. Lapses (RTs>500ms) on the PVT and target accuracy on the AX-CPTs and CPT-IP were calculated as indices of vigilant attention. Intraclass correlation coefficients (ICCs) were used to quantify the stability of individual differences, and absolute rank-order correlation (|ρ|) was used to compare the three indices.

Results

The stability of individual differences ranged from fair to substantial (PVT: ICC=0.44; AX-CPTs: ICC=0.73; CPT-IP: ICC=0.31). The rank-order correlation between the AX-CPTs and CPT-IP vigilant attention indices was relatively high (|ρ|=0.44), whereas correlations with PVT lapses were much lower (AX-CPTs: |ρ|=0.14; CPT-IP: |ρ|=0.04).

Conclusion

Individual differences during TSD were moderately stable for each index of vigilant attention, but the relationships between PVT lapses and the other indices were weak. This suggests that any or all of the indices considered here are not pure measures of vigilant attention, or that vigilant attention may constitute multiple, distinct constructs.

Support

CDMRP grant W81XWH-16-1-0319

0274 Associations of TNFα Gene Polymorphism with Resilience to Sleep Deprivation and Caffeine Sensitivity

Introduction

Sleep deprivation degrades the fidelity of human brain information processing, leading to cognitive impairment. Carriers of the A allele of a single nucleotide polymorphism of the TNFα gene (G308A, rs1800629) have been found to be resilient to cognitive impairment due to sleep deprivation as compared to individuals homozygous for the G allele. Caffeine mitigates the cognitive impairment associated with sleep deprivation. We investigated whether the effects of caffeine and TNFα genotype interact.

Methods

In an 18-day, controlled, in-laboratory study, 12 healthy adults (age 27.4±6.9; 6 females) underwent three sessions of 48-hour total sleep deprivation (TSD), with each TSD session preceded and followed by three nights of baseline and/or recovery sleep (10 hours time in bed). In randomized, counterbalanced, double-blind, placebo-controlled fashion, during each TSD session a specific dose of caffeine (0, 200, or 300 mg) was administered four times at 12-hour intervals. Vigilant attention was measured every 2 hours during each TSD session with a psychomotor vigilance test (PVT), for which the log of the signal-to-noise ratio (LSNR) derived from the RT distribution was determined as a measure of the fidelity of information processing. Each subject’s TNFα genotype was assessed from a blood sample.

Results

Subjects homozygous for the TNFα G allele showed greater PVT impairment during sleep deprivation in the 0 mg caffeine (i.e., placebo) condition as compared to carriers of the A allele and as compared to the 200 and 300 mg caffeine conditions (mixed-effects ANOVA, genotype by dose interaction: F2,566=5.23, p=0.005). There was no appreciable caffeine-related difference in performance for carriers of the A allele, who were relatively resilient to TSD regardless of caffeine dose.

Conclusion

These results suggest non-additive, interacting effects of TNFα genotype and caffeine and a potentially shared mechanism of action with regard to the fidelity of information processing during sleep deprivation.

Support

This research was supported by ONR. AJB, TJB, VFC, RHR were supported by DoD MOMRP-USAMRDC. The views expressed here are those of the authors and do not represent the official policy or position of the DoD.

0043 BDNF Genotype Modulates Circadian Response to Simulated Nightwork in IL-6

Introduction

Brain Derived Neurotrophic Factor (BDNF) is involved in synaptic plasticity and may be involved in sleep and circadian regulation. The BDNF gene has a single nucleotide polymorphism (Val66Met) resulting in decreased BDNF secretion. The Met allele is associated with reduced nocturnal slow wave activity following sleep loss, and may also enable more flexible adaptation to circadian misalignment. We used IL-6, a pleiotropic cytokine with biphasic circadian secretion, to compare the response of Val and Met carriers to circadian misalignment.

Methods

15 healthy men (26.7±4.9y) participated in a 16-day laboratory study of simulated nightwork with two nighttime duty cycles and an intervening 58h restart break. The study began with two baseline 10h time in bed (TIB) nighttime sleep opportunities (22:00-08:00) and a 5h TIB afternoon nap (15:00-20:00). Each duty cycle consisted of five nighttime waking periods and four 10h TIB daytime sleeps (10:00-20:00). The restart break began with a 5h TIB morning nap (10:00-15:00), followed by two 10h TIB nighttime sleeps (22:00-08:00). The middle two daytime sleep periods from each duty cycle were monitored polysomnographically and visually scored. During the baseline and restart periods, blood samples were collected at 08:25 and then at 2h intervals until 20:00. Plasma IL-6 was measured by means of ELISA. BDNF Val66Met genotype was determined from whole blood.

Results

The genotype distribution was in Hardy-Weinberg equilibrium (Val/Val:5; Val/Met:10; Met/Met:0). On average, Met carriers slept 52.2min longer (p=0.052) and displayed 27.7min more REM (p=0.013) than Val/Val subjects during the 10h TIB daytime sleeps. Baseline IL-6 concentrations were similar between genotypes. However, during the restart break, Val/Met subjects showed reduced IL-6 compared to baseline and to Val/Val subjects (p=0.022).

Conclusion

BDNF Val66Met heterozygotes exhibited blunted IL-6 levels following simulated nightwork, suggesting this genotype experienced a substantial circadian shift and/or altered response to sleep loss associated with daytime sleep.

Support

FMCSA award DTMC75-07-D-00006; Elliot D. Weitzman, M.D. Research Grant from the Sleep Research Society Foundation.

0332 Effect of Prior Sleep Duration on Distinct Measures of Daytime Cognitive Performance in Late Adolescence

Introduction

A recent longitudinal study of sleep need changes across adolescence reported how prior sleep duration affects daytime sleepiness and vigilant attention in children ages 10-16 years. In a follow-up study, we extend the age range in a new group of participants and add additional performance tests. Here we report year 1 data on the effect of systematically varied time in bed (TIB) on daytime vigilance, working memory, and decision making.

Methods

Data are for 52 participants aged 15.0-20.4 years (mean±SD: 17.7±1.8 years). Annually, participants keep each of three different TIB schedules: 7h, 8.5h or 10h TIB for 4 consecutive nights. The 4th night is followed by a laboratory day of performance testing. The day includes four 10-minute psychomotor vigilance tests (PVT); a serial position Sternberg working memory task; and an AX continuous performance test with switch (AX-CPTs) measuring cognitive flexibility in decision making.

Results

PVT performance evaluated by the log of the signal to noise ratio (LSNR) improved monotonically with increasing TIB (p<0.0001). TIB also affected serial position Sternberg task accuracy (p=0.008) but not the probe position effect (p=0.66), indicating that TIB did not affect working memory. TIB also affected AX-CPTs accuracy (p<0.0001), but TIB did not significantly affect decision making and cognitive flexibility measures extracted from this task (all p>0.09).

Conclusion

The initial data from this longitudinal study on older adolescents confirm what we observed for younger adolescents. Increasing TIB improves daytime vigilance but does not affect working memory. These initial results also do not indicate a TIB related improvement in decision making. Data from the entire three year longitudinal study will allow us to further investigate relations of performance to prior sleep duration and whether these relations change with age. Results from dose-response studies such as these can help guide sleep duration recommendations.

Support

PHS grant R01 HL116490 supported this work.

0730 Individuals Receiving Methadone For Medication-Assisted Treatment Of Opioid Use Disorder Show Evidence Of Respiratory Depression

Introduction

A well-established consequence of opiate use is respiratory depression during sleep, with a high prevalence of central sleep apneas. Medication-assisted treatment (MAT) is a widely used therapy for opioid use disorder (OUD) designed to reduce withdrawal symptoms and drug cravings. We investigated the presence of respiratory depression during sleep in patients receiving methadone-based opioid replacement treatment as part of a MAT program for OUD.

Methods

N=6 individuals (5 females, ages 43.8±12.8y, BMI 27.2±4.1kg/m2), who were within 90 days of methadone initiation, underwent in-laboratory overnight polysomnography (8h TIB, 22:00-06:00). Apneaic and hypopneic events were determined using AASM criteria.

Results

The average Apnea-Hypopnea Index (AHI) was 16.5±9.0 events/h, with 2 individuals exceeding the threshold of moderate sleep apnea (>15 events/h). 89.5% of the observed apnea-hypopnea events occurred during NREM sleep. Of all events, 57.1±16.3% were central apneas; and of all obstructive, central, and mixed apnea events, 93.0±14.3% were central apneas.

Conclusion

Individuals with OUD receiving methadone-based MAT may be at risk of respiratory depression during sleep, as evidenced by the frequent occurrence of central sleep apneas. Such risk could be a contributing factor in opioid overdose deaths. Currently, performing respiratory assessments during sleep is not considered standard of care in MAT programs. Our preliminary data suggest that monitoring and treatment of respiratory depression during sleep may be indicated in OUD patients on methadone-based MAT.

Support

Supported in part by a seed grant from the Washington State University Office of Research Advancement and Partnerships.

0300 The Circadian Timing of Sleep Affects the Rate of Accumulation of Neurobehavioral Impairment Across Days of Sleep Restriction

Introduction

Chronic restriction of nighttime sleep to less than ~8h/day leads to build-up of neurobehavioral impairment across days. Although it is known that sleep loss effects depend on the circadian timing of sleep, it is not known how the timing of restricted sleep influences the accumulation of neurobehavioral impairment over days. Here we studied the accumulation of impairment across days of restricted sleep placed in the morning or afternoon.

Methods

N=71 healthy young adults (39% female; ages 21-45y, mean±SD: 27.9±6.6y) completed a 14-day laboratory study. After two baseline days with nighttime sleep (8h TIB: 23:30-07:30), subjects were randomized to 10 consecutive days of A) morning sleep at 4h, 6h, or 8h TIB ending at 11:30 each day (n=18, 8, 8, respectively), or B) afternoon sleep at 4h, 6h, or 8h TIB ending at 19:30 each day (n=13, 17, 7, respectively). Subjects were tested on the 10min psychomotor vigilance test (PVT) every ~2 hours during scheduled wakefulness. Daily averages for PVT lapses (RTs>500ms) observed between 2h and 14h after awakening were analyzed with non-linear mixed-effects regression to investigate differences in the neurobehavioral impairment build-up rate between sleep restriction conditions.

Results

Afternoon sleep conditions showed a significant sleep dose-response effect (p<0.001), with the fastest accrual of PVT performance deficits across days in the 4h condition, and slow-to-negligible accumulation (p=0.36) of PVT performance deficits in the 8h condition. However, morning sleep resulted in no significant sleep dose-response effect (p=0.96). All 3 morning sleep doses displayed negligible (p≥0.12) accumulation of impairment across days.

Conclusion

In this sample of young adults, sleep dosages ending in the morning (at 11:30) appear to provide considerable protection against cumulative performance deficits from sleep restricted to 4h-6h/day over 10 days, suggesting that the afternoon circadian promotion of wakefulness can sustain behavioral alertness even over multiple days of repeated sleep restriction.

Support

NIH grants R01-NR04281 and M01-RR00040

0025 Circadian Dysregulation of DNA Repair and Increased Endogenous and Exogenous Sensitivity to DNA Damage Precipitate Elevated Cancer Risk Associated with Night Shift Work

Introduction

The elevated cancer risk associated with night shift work is poorly understood. To investigate whether circadian disruption may play a role, we assessed the circadian transcriptome and its association with hallmark cancer pathways, as well as sensitivity to endogenous and exogenous sources of DNA damage, after in-laboratory simulated shift work.

Methods

N=14 healthy humans (aged 22-34y; 10m, 4f) were exposed to a simulated night shift schedule (daytime sleep: 10:00-18:00) or a simulated day shift schedule (nighttime sleep: 22:00-06:00) for 3 days (n=7 in each condition). After the simulated shift schedule, subjects underwent a 24h constant routine protocol, during which blood was collected every 3h. Lymphocytes were extracted from the blood samples and subjected to transcriptome analysis using a NanoString multiplex assay. We evaluated 726 mRNA cancer hallmark targets (NanoString PanCancer Pathway Panel) and 17 circadian clock genes, with 18 arrhythmic internal controls. Gene expression was analyzed for circadian rhythmicity using mixed-effects cosinor analysis. Further, lymphocytes were investigated for DNA damage using an alkaline comet assay and immunofluorescence assessment of DNA damage response biomarkers BRCA1 and γH2AX. Lymphocytes collected at 07:30 and 19:30 were also exposed to ionizing radiation (2.5Gy) and DNA damage response assessments were repeated.

Results

Simulated night shift caused widespread disruption of circadian rhythmicity, as measured under constant routine, for core clock genes and the transcriptome of cancer hallmark pathways. The DNA repair pathway showed significant enrichment of rhythmic genes (p<0.05) after the simulated day shift schedule only. Following simulated night shift, lymphocytes showed induction of endogenous DNA damage, with extended tail in the comet assay (p<0.001), and higher percentage of lymphocytes with BRCA1 and γH2AX foci (p<0.01). Lymphocytes collected at 19:30 showed enhanced impact of ionizing radiation as indicated by increased prevalence of cells with BRCA1 and γH2AX foci (p<0.05).

Conclusion

Circadian dysregulation of DNA repair mechanisms and increased sensitivity to DNA damage following night shift work may increase genomic instability and precipitate elevated cancer risk in night shift workers.

Support

NIH grants ES022640 and CA227381, CDMRP award W81XWH-18-1-0100, and Pacific Northwest National Laboratory BRAVE investment under DOE contract DE-AC05-76RL01830.

0121 Effect of Total Sleep Deprivation on Word Recognition of Previously Studied Words with Different Emotional Valence

Introduction

Stimuli with an emotional valence tend to produce better recognition from memory than neutral stimuli. Sleep loss is believed to increase reactivity to negative stimuli, as compared to positive stimuli, which may comparatively enhance subsequent recognition from memory for negative stimuli. We investigated the impact of total sleep deprivation (TSD) on recognition accuracy for words with different emotional valence using the Affective Item Source Memory Task (AISM).

Methods

N=14 adults (ages 21–39; 7 females) completed a 4-day in-laboratory study with 9h baseline sleep (22:00-07:00), 39h acute TSD, and 9h recovery sleep. The AISM was administered at 16:30 during baseline and after 34h TSD. During a 5min study phase, participants heard a list, twice, of 20 positive, 20 negative, and 20 neutral words spoken with a male or female voice. During an immediately subsequent 8min recognition phase, participants heard 120 words (50% new) and judged whether each word had been presented in the study list (item memory). For words judged to have been presented previously, participants indicated whether those were presented by a female or male speaker (source memory).

Results

Mixed-effects ANOVA showed effects of session (p<0.001) and valence (p<0.001) on item memory. At baseline, accuracy was greatest for neutral and positive words compared to negative words. During TSD, accuracy declined for all three valences, and no interaction of session by valence was detected. No effects of TSD or valence on source memory were observed.

Conclusion

Sleep deprivation reduced item memory for words of all valence types. However, there was no comparatively greater impact on item or source memory for negative words nor any differential effect of TSD for different valences. Whether our results would hold with longer time intervals between task phases or an intervening sleep period remains to be determined.

Support

Jazz Pharmaceuticals

0308 DRD2 C957T Genotype Influences Vigilant Attention Performance Impairment During Total Sleep Deprivation

Introduction

There are substantial, phenotypical individual differences in the adverse impact of total sleep deprivation (TSD) on vigilant attention performance. Dopaminergic genotypes have been found to contribute to these phenotypical differences. Here we investigated the association between a single nucleotide polymorphism (SNP) of the dopamine receptor D2 (DRD2) gene, C957T (rs6277), on vigilant attention performance measured with the psychomotor vigilance test (PVT) in a laboratory TSD study.

Methods

N=46 healthy adults (ages 26.0±5.3y; 25 females) completed a 4-day in-laboratory study with a baseline day (10h time in bed: 22:00-08:00), a 38h TSD period, and a recovery day (10h time in bed: 22:00-08:00). DNA isolated from whole blood was assayed for DRD2 C957T genotype using real-time polymerase chain reaction. PVT performance was measured during TSD at 2-4h intervals, and analyzed for genotype using mixed-effects analysis of covariance of lapses of attention (RTs>500ms).

Results

The genotype distribution in this sample - 28.3% C/C, 50.0% C/T, 21.7% T/T - was found to be in Hardy-Weinberg Equilibrium (X21=0.0008, p=0.98). As expected, there was a significant effect of time awake on PVT performance (F14,602=26.67, p<0.001). There was a significant main effect of DRD2 genotype (F2,602=3.24, p=0.040) and a significant interaction of time awake by DRD2 genotype (F28,602=1.96, p=0.003). Subjects homozygous for the T allele showed greater impairment during extended wakefulness than carriers of the C allele. Genotype explained 7.6% of the variance in the PVT data observed during the 38h TSD period.

Conclusion

Subjects homozygous for the T allele of DRD2 C957T were considerably more vulnerable to TSD-induced PVT performance impairment than carriers of the C allele. A recent study showed that DRD2 C957T influences PVT performance in interaction with a SNP of the DAT1 gene. Here, DRD2 genotype was by itself also associated with PVT performance impairment during TSD.

Support

CDMRP awards W81XWH-16-1-0319 and W81XWH-18-1-0100.

1038 Sleep Architecture In Individuals Receiving Methadone For Medication-Assisted Treatment Of Opioid Use Disorder

Introduction

It has been established that the use of opioids suppresses stage N3 sleep. For individuals with opioid use disorder (OUD), medication-assisted treatment (MAT) is a widely employed opioid replacement therapy used to mitigate withdrawal effects and drug cravings. We investigated sleep architecture in individuals receiving methadone-based MAT.

Methods

N=6 individuals (aged 43.8±12.8y; 5 females), who were within 90 days of methadone initiation, underwent in-laboratory overnight polysomnography (8h TIB; 22:00-06:00). Prior to bedtime, pain intensity and opioid withdrawal symptoms were assessed using the Numeric Pain Rating Scale (0-10) and the Clinical Opiate Withdrawal Scale (0-48). Sleep recordings were scored visually according to AASM guidelines.

Results

In this sample, subjects exhibited 87.4-93.0% (M: 92.2%) sleep efficiency (SE), 8.0-16.2min (M: 12.1min) sleep latency (SL), 5.5-7.5% (M: 6.5%) N1, 46.4-52.7% (M: 49.6%) N2, 20.7-30.6% (M: 25.6%) N3, 17.5-19.1% (M: 18.3%) REM, 28.0-38.5min (M: 33.3min) N3 latency, and 84.1-125.9min (M: 105.0min) REM latency. Subjects reported moderate pain intensity scores of 5-6 (M: 5.3) and mild to moderate withdrawal symptoms of 1-15 (M: 7.8).

Conclusion

Relative to published healthy sleeper norms, subjects showed more N1 and N3 and less REM sleep. The increased N3 was unexpected given that opioids (such as methadone) typically suppress N3; it may reflect subjects carrying a substantial sleep debt. Pain and withdrawal symptoms may be a factor increasing N1 and reducing REM sleep. Such potential sleep deficiencies may interfere with subjects achieving OUD recovery goals and are worthy of further investigation.

Support

Supported in part by a seed grant from the Washington State University Office of Research Advancement and Partnerships.

Temperature profiles, and the effect of sleep on them, in relation to morningness-eveningness in healthy female subjects

There were 15 healthy female subjects, differing in their position on the "morningness-eveningness" scale, studied for 7 consecutive days, first while living a sedentary lifestyle and sleeping between midnight and 08:00 and then while undergoing a "constant routine." Rectal temperature was measured at regular intervals throughout this time, and the results were subjected to cosinor analysis both before and after "purification" for the effects of physical activity. Results showed that there was a phase difference in the circadian rhythm of core temperature that was associated with the morningness score, with calculations that "morning types" would be phased earlier than "evening types" by up to about 3 h. This difference in phase (which was also statistically significant when the group was divided by a median split into a "morning group" and an "evening group") could not be attributed to effects of waking activity and existed in spite of the subjects keeping the same sleep-wake schedule. Moreover, it persisted when the subjects' data had been purified and when the data were obtained from the constant routine. That is, there was an endogenous component to this difference in phase of the core temperature. The morning group also showed a greater fall of core temperature during sleep; this was assessed in two ways, the main one being a comparison of constant routine and nychthemeral data sets after correction for any effects of activity. Even though the morning group was sleeping at a later phase of their circadian temperature rhythm than was the evening group, neither group showed a fall of temperature due to sleep that varied with time elapsed since the temperature acrophase. It is concluded that another factor that differs between morning and evening types is responsible for this difference.