Sex difference in the near-24-hour intrinsic period of the human circadian timing system.

The circadian rhythms of melatonin and body temperature are set to an earlier hour in women than in men, even when the women and men maintain nearly identical and consistent bedtimes and wake times. Moreover, women tend to wake up earlier than men and exhibit a greater preference for morning activities than men. Although the neurobiological mechanism underlying this sex difference in circadian alignment is unknown, multiple studies in nonhuman animals have demonstrated a sex difference in circadian period that could account for such a difference in circadian alignment between women and men. Whether a sex difference in intrinsic circadian period in humans underlies the difference in circadian alignment between men and women is unknown. We analyzed precise estimates of intrinsic circadian period collected from 157 individuals (52 women, 105 men; aged 18-74 y) studied in a month-long inpatient protocol designed to minimize confounding influences on circadian period estimation. Overall, the average intrinsic period of the melatonin and temperature rhythms in this population was very close to 24 h [24.15 ± 0.2 h (24 h 9 min ± 12 min)]. We further found that the intrinsic circadian period was significantly shorter in women [24.09 ± 0.2 h (24 h 5 min ± 12 min)] than in men [24.19 ± 0.2 h (24 h 11 min ± 12 min); P < 0.01] and that a significantly greater proportion of women have intrinsic circadian periods shorter than 24.0 h (35% vs. 14%; P < 0.01). The shorter average intrinsic circadian period observed in women may have implications for understanding sex differences in habitual sleep duration and insomnia prevalence.

Circadian- and wake-dependent influences on face-name memory in healthy men and women over 3weeks of chronic sleep restriction.

OBJECTIVES: Facial recognition is one of the key functions of the human brain, and linking a face to a name is critical in many social and occupational settings. This study assessed circadian- and wake-dependent effects on face-name recognition in healthy adults. METHODS: Thirteen healthy adults (20-70years; 7 F) were studied in a 39-day inpatient protocol that included 3weeks of 28 hours forced desynchrony with sleep restriction (6.5:21.5 hours sleep:wake). Starting 3 hours after scheduled wake, 6 novel face-name pairs were presented every 4 waking hours; recognition was tested 2 hours later. Performance data were averaged across ∼4 hours circadian phase or time-awake bins. RESULTS: Face-name recognition deteriorated with increased time awake (p < .0001) and exhibited significant circadian variation (p < .0001), with worst performance shortly after the core temperature nadir. There was a significant interaction between sex and circadian phase (p = .0177), with women performing significantly better than men at all circadian phases except 60° and 120°. Women exhibited a significantly higher amplitude than men during the third week of forced desynchrony (p < .01). CONCLUSIONS: Like many other aspects of neurobehavioral performance, recalling face-name associations is impacted by both duration of time awake and circadian phase. These results have implications for face recognition testing in medical contexts, such as in testing for dementia, because performance may be impacted by sleep deficiency and the time of testing.

The effects of circadian phase, time awake, and imposed sleep restriction on performing complex visual tasks: evidence from comparative visual search.

Cognitive performance not only differs between individuals, but also varies within them, influenced by factors that include sleep-wakefulness and biological time of day (circadian phase). Previous studies have shown that both factors influence accuracy rather than the speed of performing a visual search task, which can be hazardous in safety-critical tasks such as air-traffic control or baggage screening. However, prior investigations used simple, brief search tasks requiring little use of working memory. In order to study the effects of circadian phase, time awake, and chronic sleep restriction on the more realistic scenario of longer tasks requiring the sustained interaction of visual working memory and attentional control, the present study employed two comparative visual search tasks. In these tasks, participants had to detect a mismatch between two otherwise identical object distributions, with one of the tasks (mirror task) requiring an additional mental image transformation. Time awake and circadian phase both had significant influences on the speed, but not the accuracy of task performance. Over the course of three weeks of chronic sleep restriction, speed but not accuracy of task performance was impacted. The results suggest measures for safer performance of important tasks and point out the importance of minimizing the impact of circadian phase and sleep-wake history in laboratory vision experiments.

Young adults are more vulnerable to chronic sleep deficiency and recurrent circadian disruption than older adults.

More than a third of US adults report fewer than 6 hours of sleep a night, making chronic sleep restriction a growing public health concern. Sleep curtailment is associated with an increase in industrial accidents, motor vehicle accidents, medical and other occupational errors. Young adults are more vulnerable to acute sleep deprivation than older adults, but less is known about how young vs. older adults respond to the more commonly experienced chronic sleep restriction. To test the hypothesis that young adults are more vulnerable to chronic sleep loss than older adults, we compared data from young and older adults who underwent three weeks of chronic sleep restriction (equivalent to 5.6 hours/24 hours) combined with recurrent circadian disruption in an experiment that enabled us to separate the influences of the sleep-wake homeostatic process, the circadian timing system, and the chronic sleep deficit. We found that while young and older adults reported similar levels of subjective sleepiness, objective measures of sleepiness revealed that young adults were more vulnerable and had more attentional failures than the older adults. These results have important public health implications, particularly related to prevention of sleep-related motor vehicle crashes in young drivers. Further research is needed to understand the neurobiological basis of these age-related differences.

Principal component structuring of the non-REM Sleep EEG spectrum in older adults yields age-related changes in the sleep and wake drives.

Age-related disturbances of the sleep-wake cycle can reflect ontogenetic changes in regulatory mechanisms underlying normal and pathological aging, but the exact nature of these changes remains unclear. The present report is the first attempt to apply principal component analysis to the electroencephalographic (EEG) spectrum to examine of whether the observed age-related changes in the objective sleep measures can be linked to the opponent sleep-promoting and wake-promoting processes. The EEG indicators of these processes--scores on the 1st and 2nd principal components of the EEG spectrum, respectively--were compared in 15 older (57-74 years) and 16 younger (20-31 years) healthy volunteers. The scores were calculated for non-REM sleep episodes which occurred during ten 75-min naps scheduled every 150 min throughout a 40-h constant routine protocol. Both, a decrease of the 1st principal component score and an increase of the 2nd principal component score were found to contribute to such most obvious age-related modification of the sleep EEG spectrum as attenuation of EEG slow-wave activity in older people. Therefore, we concluded that the normal aging process can reflect both a weakening of the sleep-promoting process and a strengthening of the wake-promoting process, respectively. Such bidirectional changes in chronoregulatory processes may explain why sleep of older people is characterized by the few profitable and a number of detrimental features (i.e., a better ability to cope with daytime sleepiness and sleep loss vs. difficulty of falling asleep, decreased total nighttime sleep, "lightened" and fragmentized sleep, unwanted early morning awakenings, etc.).