The Relation of Shift Work Tolerance to the Circadian Adjustment

The amplitude and phasing of circadian rhythms are under discussion as possible predictors of tolerance to night work. In a field study, subjective sleepiness and oral temperature of 147 female nurses were measured at 2-hour intervals during a period with one morning shift and two consecutive night shifts. The nurses also filled out a questionnaire. Two types of tolerance indices were constructed: The “health index” was based on questions referring to general fatigue, gastrointestinal symptoms, and sleep disturbances, and the “sleepiness index” on the actual subjective ratings of sleepiness. According to the health index, the group with good tolerance had a larger circadian amplitude of the oral temperature rhythm on the day of the morning shift than the group with poor tolerance. However, with regard to the sleepiness index, the corresponding difference between the groups with good or poor tolerance was not significant. The data did not confirm the hypothesis that predicts a quick adjustment of the circadian rhythm when the circadian amplitude is small before the change to night work. The contradictory results found in this and in other studies do not yet permit prediction of tolerance to night work.     

Individual differences in subjective and objective alertness during sleep deprivation are stable and unrelated

This study examines the individual reproducibility of alterations of subjective, objective, and EEG measures of alertness during 27 h of continuous wakefulness and analyzes their interrelationships. Eight subjects were studied twice under similar constant-routine conditions. Scales and performance tasks were administered at hourly intervals to define temporal changes in subjective and objective alertness. The wake EEG was recorded every 2 h, 2 min with eyes open and 2 min with eyes closed. Plasma glucose and melatonin levels were measured to estimate brain glucose utilization and individual circadian phase, respectively. Decrements of subjective alertness and performance deficits were found to be highly reproducible for a given individual. Remarkably, there was no relationship between the impairments of subjective and objective alertness. With increased duration of wakefulness, EEG activity with eyes closed increased in the delta range and decreased in the alpha range, but the magnitudes of these changes were also unrelated. These findings indicate that sleep deprivation has highly reproducible, but independent, effects on brain mechanisms controlling subjective and objective alertness.     

Level of alpha amylase activity in human saliva as a non-invasive biochemical marker of sleep deprivation

To investigate the usefulness of the enzyme salivary alpha amylase as a biochemical marker of sleep deprivation in human subjects. Total 168 healthy school-going adolescents studying in 9th grade were selected randomly from morning shift (n = 84) and dayshift (n = 84) schools. The study was undertaken longitudinally for a period of 2 years. Study encompassed administration of questionnaire and collection of saliva samples from the participants. Activity of salivary alpha amylase (sAA) activity was estimated spectrophotometrically and statistical analysis was performed to determine the association between sAA activity and sleep duration. Excessive daytime sleepiness among students was also studied in association with sAA activity. sAA activity of students was found to have a negative correlation with the duration of sleep and a positive correlation with their level of sleepiness. Morning shift students were found to have significantly less sleep and correspondingly higher sAA activity as compared to dayshift students. A significant increase in the sAA activity was noticed in the second year as the students progressed from 9th to 10th grade. Higher amylase activity was also observed in sleep deprived students suffering from excessive daytime sleepiness irrespective of school timings. Salivary alpha amylase activity increases in saliva in response to sleep deprivation. School timings may modulate sleep duration of students. Present finding reveals that sAA could be an appropriate non-invasive biochemical marker for the objective assessment of sleep deprivation among individuals as well as at population level.

     

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 3h. 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. (Chronobiology International, 18(2), 227-247, 2001)     

Role of morning melatonin administration and attenuation of sunlight exposure in improving adaptation of night-shift workers

The authors studied whether melatonin administration improves adaptation of workers to nightshift and if its beneficial effect is enhanced by attenuation of morning sunlight exposure. Twelve nightshift nurses received three treatments: Placebo (Pla), Melatonin (Mel), and Melatonin with Sunglasses (Mel-S). Each treatment procedure was administered for 2 d of different 4d nightshifts in a repeated measures crossover design. In Pla, nurses were treated with placebo before daytime sleep and allowed exposure to morning sunlight. In Mel, 6 mg of melatonin was similarly administered before daytime sleep with morning sunlight permitted. In Mel-S, 6 mg of melatonin was given as in Mel, with sunglasses worn in the morning to attenuate sunlight exposure. Placebo or melatonin was administered during days 2 and 3 when the first and second daytime sleep occurred. Nocturnal alertness and performance plus daytime sleep and mood states were assessed during all three treatments. The sleep period and total sleep times were significantly increased by melatonin treatments; yet, nocturnal alertness was only marginally improved. There were no differences between Mel and Mel-S. Performance tests revealed no difference between Pla and melatonin treatments. Melatonin exerted modest benefit in improving the adaptation of workers to nightshift, and its effect was not enhanced by attenuation of morning sunlight exposure.     

Quantification of sleepiness through principal component analysis of the electroencephalographic spectrum

Although circadian and sleep research has made extraordinary progress in the recent years, one remaining challenge is the objective quantification of sleepiness in individuals suffering from sleep deprivation, sleep restriction, and excessive somnolence. The major goal of the present study was to apply principal component analysis to the wake electroencephalographic (EEG) spectrum in order to establish an objective measure of sleepiness. The present analysis was led by the hypothesis that in sleep-deprived individuals, the time course of self-rated sleepiness correlates with the time course score on the 2nd principal component of the EEG spectrum. The resting EEG of 15 young subjects was recorded at 2-h intervals for 32-50 h. Principal component analysis was performed on the sets of 16 single-Hz log-transformed EEG powers (1-16 Hz frequency range). The time course of self-perceived sleepiness correlated strongly with the time course of the 2nd principal component score, irrespective of derivation (frontal or occipital) and of analyzed section of the 7-min EEG record (2-min section with eyes open or any of the five 1-min sections with eyes closed). This result indicates the possibility of deriving an objective index of physiological sleepiness by applying principal component analysis to the wake EEG spectrum.     

Quantification of Sleepiness Through Principal Component Analysis of the Electroencephalographic Spectrum

Although circadian and sleep research has made extraordinary progress in the recent years, one remaining challenge is the objective quantification of sleepiness in individuals suffering from sleep deprivation, sleep restriction, and excessive somnolence. The major goal of the present study was to apply principal component analysis to the wake electroencephalographic (EEG) spectrum in order to establish an objective measure of sleepiness. The present analysis was led by the hypothesis that in sleep-deprived individuals, the time course of self-rated sleepiness correlates with the time course score on the 2nd principal component of the EEG spectrum. The resting EEG of 15 young subjects was recorded at 2-h intervals for 32–50?h. Principal component analysis was performed on the sets of 16 single-Hz log-transformed EEG powers (1–16?Hz frequency range). The time course of self-perceived sleepiness correlated strongly with the time course of the 2nd principal component score, irrespective of derivation (frontal or occipital) and of analyzed section of the 7-min EEG record (2-min section with eyes open or any of the five 1-min sections with eyes closed). This result indicates the possibility of deriving an objective index of physiological sleepiness by applying principal component analysis to the wake EEG spectrum. (Author correspondence: putilov@ngs.ru)     

ROLE OF MORNING MELATONIN ADMINISTRATION AND ATTENUATION OF SUNLIGHT EXPOSURE IN IMPROVING ADAPTATION OF NIGHT-SHIFT WORKERS

The authors studied whether melatonin administration improves adaptation of workers to nightshift and if its beneficial effect is enhanced by attenuation of morning sunlight exposure. Twelve nightshift nurses received three treatments: Placebo (Pla), Melatonin (Mel), and Melatonin with Sunglasses (Mel-S). Each treatment procedure was administered for 2 d of different 4d nightshifts in a repeated measures crossover design. In Pla, nurses were treated with placebo before daytime sleep and allowed exposure to morning sunlight. In Mel, 6 mg of melatonin was similarly administered before daytime sleep with morning sunlight permitted. In Mel-S, 6 mg of melatonin was given as in Mel, with sunglasses worn in the morning to attenuate sunlight exposure. Placebo or melatonin was administered during days 2 and 3 when the first and second daytime sleep occurred. Nocturnal alertness and performance plus daytime sleep and mood states were assessed during all three treatments. The sleep period and total sleep times were significantly increased by melatonin treatments; yet, nocturnal alertness was only marginally improved. There were no differences between Mel and Mel-S. Performance tests revealed no difference between Pla and melatonin treatments. Melatonin exerted modest benefit in improving the adaptation of workers to nightshift, and its effect was not enhanced by attenuation of morning sunlight exposure.     

Meta-Analysis of the Relationship Between Total Sleep Deprivation and Performance

Studies consistently show that total sleep deprivation (TSD) and measures of performance are negatively correlated. However, an accurate quantitative summary of the relationship between these variables has not yet been reported. After collection of the data from 27 relevant studies, meta-analytic techniques were used to test several hypotheses. The correlations were found to be highest for TSD of ≥45 h, speed rather than accuracy measures of performance, and work-paced rather than self-paced tasks. These findings are consistent with the “lapse hypothesis” that posits microsleeps during long hours of sleep deprivation.     

Sleep homeostasis and models of sleep regulation

According to the two-process model of sleep regulation, the timing and structure of sleep are determined by the interaction of a homeostatic and a circadian process. The original qualitative model was elaborated to quantitative versions that included the ultradian dynamics of sleep in relation to the non-REM-REM sleep cycle. The time course of EEG slow-wave activity, the major marker of non-REM sleep homeostasis, as well as daytime alertness were simulated successfully for a considerable number of experimental protocols. They include sleep after partial sleep deprivation and daytime napping, sleep in habitual short and long sleepers, and alertness in a forced desynchrony protocol or during an extended photoperiod. Simulations revealed that internal desynchronization can be obtained for different shapes of the thresholds. New developments include the analysis of the waking EEG to delineate homeostatic and circadian processes, studies of REM sleep homeostasis, and recent evidence for local, use-dependent sleep processes. Moreover, nonlinear interactions between homeostatic and circadian processes were identified. In the past two decades, models have contributed considerably to conceptualizing and analyzing the major processes underlying sleep regulation, and they are likely to play an important role in future advances in the field.     

Effect of bright light on EEG activities and subjective sleepiness to mental task during nocturnal sleep deprivation

The purpose of this study was to investigate the effect of the exposure to bright light on EEG activity and subjective sleepiness at rest and at the mental task during nocturnal sleep deprivation. Eight male subjects lay awake in semi-supine in a reclining seat from 21:00 to 04:30 under the bright (BL; >2500 lux) or the dim (DL; <150 lux) light conditions. During the sleep deprivation, the mental task (Stroop color-word conflict test: CWT) was performed each 15 min in one hour. EEG, subjective sleepiness, rectal and mean skin temperatures and urinary melatonin concentrations were measured. The subjective sleepiness increased with time of sleep deprivation during both rest and CWT under the DL condition. The exposure to bright light delayed for 2 hours the increase in subjective sleepiness at rest and suppressed the increase in that during CWT. The bright light exposure also delayed the increase in the theta and alpha wave activities in EEG at rest. In contrast, the effect of the bright light exposure on the theta and alpha wave activities disappeared by CWT. Additionally, under the BL condition, the entire theta activity during CWT throughout nocturnal sleep deprivation increased significantly from that in a rest condition. Our results suggest that the exposure to bright light throughout nocturnal sleep deprivation influences the subjective sleepiness during the mental task and the EEG activity, as well as the subjective sleepiness at rest. However, the effect of the bright light exposure on the EEG activity at the mental task diminishes throughout nocturnal sleep deprivation.     

Age-related changes in the circadian and homeostatic regulation of human sleep

The reduction of electroencephalographic (EEG) slow-wave activity (SWA) (EEG power density between 0.75-4.5 Hz) and spindle frequency activity, together with an increase in involuntary awakenings during sleep, represent the hallmarks of human sleep alterations with age. It has been assumed that this decrease in non-rapid eye movement (NREM) sleep consolidation reflects an age-related attenuation of the sleep homeostatic drive. To test this hypothesis, we measured sleep EEG characteristics (i.e., SWA, sleep spindles) in healthy older volunteers in response to high (sleep deprivation protocol) and low sleep pressure (nap protocol) conditions. Despite the fact that the older volunteers had impaired sleep consolidation and reduced SWA levels, their relative SWA response to both high and low sleep pressure conditions was similar to that of younger persons. Only in frontal brain regions did we find an age-related diminished SWA response to high sleep pressure. On the other hand, we have clear evidence that the circadian regulation of sleep during the 40 h nap protocol was changed such that the circadian arousal signal in the evening was weaker in the older study participants. More sleep occurred during the wake maintenance zone, and subjective sleepiness ratings in the late afternoon and evening were higher than in younger participants. In addition, we found a diminished melatonin secretion and a reduced circadian modulation of REM sleep and spindle frequency-the latter was phase-advanced relative to the circadian melatonin profile. Therefore, we favor the hypothesis that age-related changes in sleep are due to weaker circadian regulation of sleep and wakefulness. Our data suggest that manipulations of the circadian timing system, rather than the sleep homeostat, may offer a potential strategy to alleviate age-related decrements in sleep and daytime alertness levels.     

Three dimensions of individual variation in phase angle between sleep timing and timing of nocturnal rise of the feeling of sleepiness

Studies of melatonin and body temperature rhythms revealed that women, younger adults, and morning-oriented types show a relatively larger phase angle between entrained circadian phase and sleep timing than men, older adults, and evening-oriented types, respectively. However, none of these studies has been designed to compare participants representing all these three dimensions of individual variation. Since daily fluctuations in self-reported level of alertness–sleepiness closely follow the circadian rhythms of melatonin and body temperature, one can predict that a study of circadian phase characteristics of fluctuations of sleepiness shell reveals identical sex-, age-, and diurnal type-related differences in phase angle between circadian phase and sleep timing. Analysis of self-scorings of alertness–sleepiness provided by 130 healthy participants of sleep deprivation experiments confirmed this prediction. It seems that both fundamental research and field studies of sleep-deprived individuals can benefit from the evaluation of circadian phase through self-assessment of nocturnal rise of alertness–sleepiness.     

Aircrew fatigue in trans-Atlantic morning and evening flights

The aim of the investigation was to compare sleepiness and sleep on westward morning and evening flights. Seven morning-crew pilots and seven evening-crew pilots participated. Data were collected before, during, and after outward-bound (westward) and homeward-bound (eastward) flights across six time zones. A sleep/wake diary (with repeated sleepiness and performance ratings) and wrist actigraphy were used for data collection. Maximum sleep was obtained after return and minimum sleep before the outward-bound flights. Actigraphy measures and sleep efficiency over the course of the study showed no significant differences between the morning and evening crews. There was a trend for a significant effect of morning vs. evening flight for time with heavy eyelids, with the homeward-bound flight showing more time with heavy eyelids. There were no significant differences between morning and evening crews with regard to napping during the flight. The duration of wakefulness was longer for the evening flight crew. There were significant interactions for Karolinska sleepiness scale (KSS) self-ratings on both the outward-bound and homeward-bound flights, and KSS was elevated during a considerable portion of the evening flights. Rated performance showed a significant time effect, but there was no difference in self-ratings between morning and evening crews. Evening flights involve higher levels of sleepiness than morning flights, presumably because of the close proximity in time to the circadian trough of alertness.     

Aircrew Fatigue in Trans‐Atlantic Morning and Evening Flights

The aim of the investigation was to compare sleepiness and sleep on westward morning and evening flights. Seven morning‐crew pilots and seven evening‐crew pilots participated. Data were collected before, during, and after outward‐bound (westward) and homeward‐bound (eastward) flights across six time zones. A sleep/wake diary (with repeated sleepiness and performance ratings) and wrist actigraphy were used for data collection. Maximum sleep was obtained after return and minimum sleep before the outward‐bound flights. Actigraphy measures and sleep efficiency over the course of the study showed no significant differences between the morning and evening crews. There was a trend for a significant effect of morning vs. evening flight for time with heavy eyelids, with the homeward‐bound flight showing more time with heavy eyelids. There were no significant differences between morning and evening crews with regard to napping during the flight. The duration of wakefulness was longer for the evening flight crew. There were significant interactions for Karolinska sleepiness scale (KSS) self‐ratings on both the outward‐bound and homeward‐bound flights, and KSS was elevated during a considerable portion of the evening flights. Rated performance showed a significant time effect, but there was no difference in self‐ratings between morning and evening crews. Evening flights involve higher levels of sleepiness than morning flights, presumably because of the close proximity in time to the circadian trough of alertness.     

The Association between Daytime Napping and Cognitive Functioning in Chronic Fatigue Syndrome

Objectives

The precise relationship between sleep and physical and mental functioning in chronic fatigue syndrome (CFS) has not been examined directly, nor has the impact of daytime napping. This study aimed to examine self-reported sleep in patients with CFS and explore whether sleep quality and daytime napping, specific patient characteristics (gender, illness length) and levels of anxiety and depression, predicted daytime fatigue severity, levels of daytime sleepiness and cognitive functioning, all key dimensions of the illness experience.

Methods

118 adults meeting the 1994 CDC case criteria for CFS completed a standardised sleep diary over 14 days. Momentary functional assessments of fatigue, sleepiness, cognition and mood were completed by patients as part of usual care. Levels of daytime functioning and disability were quantified using symptom assessment tools, measuring fatigue (Chalder Fatigue Scale), sleepiness (Epworth Sleepiness Scale), cognitive functioning (Trail Making Test, Cognitive Failures Questionnaire), and mood (Hospital Anxiety and Depression Scale).

Results

Hierarchical Regressions demonstrated that a shorter time since diagnosis, higher depression and longer wake time after sleep onset predicted 23.4% of the variance in fatigue severity (p <.001). Being male, higher depression and more afternoon naps predicted 25.6% of the variance in objective cognitive dysfunction (p <.001). Higher anxiety and depression and morning napping predicted 32.2% of the variance in subjective cognitive dysfunction (p <.001). When patients were classified into groups of mild and moderate sleepiness, those with longer daytime naps, those who mainly napped in the afternoon, and those with higher levels of anxiety, were more likely to be in the moderately sleepy group.

Conclusions

Napping, particularly in the afternoon is associated with poorer cognitive functioning and more daytime sleepiness in CFS. These findings have clinical implications for symptom management strategies.     

Accounting for partial sleep deprivation and cumulative sleepiness in the Three-Process Model of alertness regulation

Mathematical models designed to predict alertness or performance have been developed primarily as tools for evaluating work and/or sleep-wake schedules that deviate from the traditional daytime orientation. In general, these models cope well with the acute changes resulting from an abnormal sleep but have difficulties handling sleep restriction across longer periods. The reason is that the function representing recovery is too steep--usually exponentially so--and with increasing sleep loss, the steepness increases, resulting in too rapid recovery. The present study focused on refining the Three-Process Model of alertness regulation. We used an experiment with 4 h of sleep/night (nine participants) that included subjective self-ratings of sleepiness every hour. To evaluate the model at the individual subject level, a set of mixed-effect regression analyses were performed using subjective sleepiness as the dependent variable. These mixed models estimate a fixed effect (group mean) and a random effect that accounts for heterogeneity between participants in the overall level of sleepiness (i.e., a random intercept). Using this technique, a point was sought on the exponential recovery function that would explain maximum variance in subjective sleepiness by switching to a linear function. The resulting point explaining the highest amount of variance was 12.2 on the 1-21 unit scale. It was concluded that the accumulation of sleep loss effects on subjective sleepiness may be accounted for by making the recovery function linear below a certain point on the otherwise exponential function.     

Sleep and Circadian Rhythms of Temperature and Urinary Excretion on a 22.8 hr “Day”

Two groups of subjects (total N = 6) were studied in an isolation chamber for a period of 3 weeks whilst living on a 22.8 hr “day”. Regular samples of urine were taken when the subjects were awake, deep body temperature was recorded continuously and polygraphic EEG recordings were made of alternate sleeps. The excretion in the urine of potassium, sodium, phosphate, calcium and a metabolite of melatonin were estimated.

Measurements of the quantity and quality of sleep were made together with assessments of the temperature profiles associated with sleep. In addition, cosinor analysis of circadian rhythmicity in urinary variables and temperature was performed.

The 22.8 hr “days” affected variables and subjects differently. These differences were interpreted as indicating that the endogenous component of half the subjects adjusted to the 22.8 hr “days” but that, for the other three, adjustment did not occur. When the behaviour of different variables was considered then some (including urinary potassium and melatonin, sleep length and REM sleep) appeared to possess a larger endogenous component than others (for example, urinary sodium, phosphate and calcium), with rectal temperature behaving in an intermediate manner. In addition, a comparison between different rhythms in any subject enabled inferences to be drawn regarding any links (or lack of them) that might exist between the rhythms. In this respect also, there was a considerable range in the results and no links between any of the rhythms appeared to exist in the group of subjects as a whole.

Two further groups (total N=8) were treated similarly except that the chamber clock ran at the correct rate. In these subjects, circadian rhythms of urinary excretion and deep body temperature (sleep stages and urinary melatonin were not measured) gave no evidence for deterioration. We conclude, therefore, that the results on the 22.8 hr “day” were directly due to the abnormal “day” length rather than to a prolonged stay in the isolation chamber.     

Effect of melatonin on sleep quality of COPD intensive care patients: a pilot study

Sleep deprivation is extremely common in the intensive care unit (ICU), and this lack of sleep is associated with low melatonin secretion. The objective of the current study was to explore the effect of exogenous melatonin administration on sleep quality in patients hospitalized in the pulmonary intensive care unit (ICU). We performed a double-blind, placebo-controlled study in the pulmonary ICU of a tertiary care hospital. Eight adult patients hospitalized in the pulmonary ICU with respiratory failure caused by exacerbation of chronic obstructive pulmonary disease (COPD) or with pneumonia were studied. Patients received either 3 mg of controlled-release melatonin or a placebo at 22:00, and sleep quality was evaluated by wrist actigraphy. Treatment with controlled-release melatonin dramatically improved both the duration and quality of sleep in this group of patients. Our results suggest that melatonin administration to patients in intensive care units may be indicated as a treatment for sleep induction and resynchronization of the “biologic clock.” This treatment may also help in the prevention of the “ICU syndrome” and accelerate the healing process. (Chronobiology International, 17(1), 71-76, 2000)     

Time course of neurobehavioral alertness during extended wakefulness in morning- and evening-type healthy sleepers

The aim of the study was to evaluate the influence of chronotype (morning-type versus evening-type) living in a fixed sleep-wake schedule different from one's preferred sleep schedules on the time course of neurobehavioral performance during controlled extended wakefulness. The authors studied 9 morning-type and 9 evening-type healthy male subjects (21.4 ± 1.9 yrs). Before the experiment, all participants underwent a fixed sleep-wake schedule mimicking a regular working day (bedtime: 23:30 h; wake time: 07:30 h). Then, following two nights in the laboratory, both chronotypes underwent a 36-h constant routine, performing a cognitive test of sustained attention every hour. Core body temperature, salivary melatonin secretion, objective alertness (maintenance of wakefulness test), and subjective sleepiness (visual analog scale) were also assessed. Evening-types expressed a higher level of subjective sleepiness than morning types, whereas their objective levels of alertness were not different. Cognitive performance in the lapse domain remained stable during the normal waking day and then declined during the biological night, with a similar time course for both chronotypes. Evening types maintained optimal alertness (i.e., 10% fastest reaction time) throughout the night, whereas morning types did not. For both chronotypes, the circadian performance profile was correlated with the circadian subjective somnolence profile and was slightly phase-delayed with melatonin secretion. Circadian performance was less correlated with circadian core body temperature. Lapse domain was phase-delayed with body temperature (2-4 h), whereas optimal alertness was slightly phase-delayed with body temperature (1 h). These results indicate evening types living in a fixed sleep-wake schedule mimicking a regular working day (different from their preferred sleep schedules) express higher subjective sleepiness but can maintain the same level of objective alertness during a normal waking day as morning types. Furthermore, evening types were found to maintain optimal alertness throughout their nighttime, whereas morning types could not. The authors suggest that evening-type subjects have a higher voluntary engagement of wake-maintenance mechanisms during extended wakefulness due to adaptation of their sleep-wake schedule to social constraints.