CIRCADIAN RHYTHMS IN HEALTHY AGING—EFFECTS DOWNSTREAM FROM THE PACEMAKER

Using both previously published findings and entirely new data, we present evidence in support of the argument that the circadian dysfunction of advancing age in the healthy human is primarily one of failing to transduce the circadian signal from the circadian timing system (CTS) to rhythms “downstream” from the pacemaker rather than one of failing to generate the circadian signal itself. Two downstream rhythms are considered: subjective alertness and objective performance. For subjective alertness, we show that in both normal nychthemeral (24h routine, sleeping at night) and unmasking (36h of constant wakeful bed rest) conditions, advancing age, especially in men, leads to flattening of subjective alertness rhythms, even when circadian temperature rhythms are relatively robust. For objective performance, an unmasking experiment involving manual dexterity, visual search, and visual vigilance tasks was used to demonstrate that the relationship between temperature and performance is strong in the young, but not in older subjects (and especially not in older men). (Chronobiology International, 17(3), 355–368, 2000)     

Prediction of Vigilant Attention and Cognitive Performance Using Self-Reported Alertness,Circadian Phase,Hours since Awakening,and Accumulated Sleep Loss

Sleep restriction causes impaired cognitive performance that can result in adverse consequences in many occupational settings. Individuals may rely on self-perceived alertness to decide if they are able to adequately perform a task. It is therefore important to determine the relationship between an individual’s self-assessed alertness and their objective performance, and how this relationship depends on circadian phase, hours since awakening, and cumulative lost hours of sleep. Healthy young adults (aged 18–34) completed an inpatient schedule that included forced desynchrony of sleep/wake and circadian rhythms with twelve 42.85-hour “days” and either a 1:2 (n = 8) or 1:3.3 (n = 9) ratio of sleep-opportunity:enforced-wakefulness. We investigated whether subjective alertness (visual analog scale), circadian phase (melatonin), hours since awakening, and cumulative sleep loss could predict objective performance on the Psychomotor Vigilance Task (PVT), an Addition/Calculation Test (ADD) and the Digit Symbol Substitution Test (DSST). Mathematical models that allowed nonlinear interactions between explanatory variables were evaluated using the Akaike Information Criterion (AIC). Subjective alertness was the single best predictor of PVT, ADD, and DSST performance. Subjective alertness alone, however, was not an accurate predictor of PVT performance. The best AIC scores for PVT and DSST were achieved when all explanatory variables were included in the model. The best AIC score for ADD was achieved with circadian phase and subjective alertness variables. We conclude that subjective alertness alone is a weak predictor of objective vigilant or cognitive performance. Predictions can, however, be improved by knowing an individual’s circadian phase, current wake duration, and cumulative sleep loss.     

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.     

Phase-dependent effect of room light exposure in a 5-h advance of the sleep-wake cycle: implications for jet lag

The acute disruption in sleep quality, vigilance levels, and cognitive and athletic performance observed after transmeridian flights is presumed to be the result of a transient misalignment between the endogenous circadian pacemaker and the shifted sleep schedule. Several laboratory and field experiments have demonstrated that exposure to bright artificial light can accelerate circadian entrainment to a shifted sleep-wake schedule. In the present study, the authors investigated whether the schedule of exposure to indoor room light, to which urban dwellers are typically exposed, can substantially affect circadian adaptation to a simulated eastward voyage. We enrolled 15 healthy young men in a laboratory simulation of a Montreal-to-London voyage. Subjects were exposed to 6 h of room light (mean +/- SD: 379+/-10) prior to bedtime (n = 7) or when on a progressively advancing schedule (n = 8) early in the day. The remaining 10 hours of wakefulness were spent in dim light (4+/-1 lux). Circadian assessments, performed via the constant routine procedure, evaluated the phase of the endogenous circadian rhythms of core body temperature and plasma melatonin before and after 1 week on the shifted schedule. At the end of the study, only subjects exposed to room light on the advancing schedule expressed oscillations of the endogenous circadian pacemaker in phase with the new sleep-wake cycle. In this group, a mean advance shift of the nadir of core body temperature of +5:22+/-0:15 h was observed, with parallel shifts in plasma melatonin concentration and subjective alertness. The circadian rhythms of subjects exposed to room light later in the day remained much more adjusted to the departure than to the destination time zone. These results demonstrate that the schedule of exposure to room light can substantially affect circadian adaptation to a shifted sleep-wake schedule.     

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. (Author correspondence: jack.taillard@gmail.com)     

Chronobiotic effects of the melatonin agonist LY 156735 following a simulated 9h time shift: results of a placebo-controlled trial

Introduction: The melatonin agonist LY 156735 (LY) is a new investigational drug under development to treat circadian rhythm disorders. The present study assessed the efficacy of LY to alleviate the symptoms of shift lag and to enhance readaptation of desynchronized circadian rhythms to a new time zone.

Subjects and methods: Eight healthy male volunteers of age 25-35 yr participated in three identical trials of 13d duration in a temporal isolation unit separated by washout intervals. A high dose (HD) of 5 mg and a low dose (LD) of 0.5 mg of LY and placebo (PL) were administered double-blinded in a three-period cross-over design. Each trial consisted of an adaptation period, a pre-shift period for baseline measurements, a simulated 9h phase-advance shift, and a post-shift period for follow-up. The time shift was performed at 23:00h of day 6 by advancing the laboratory time to 08:00h of day 7. Double-blind study medication was administered at 14:30h on day 6, and at 22:30h on days 7-10. Subjective ratings of jet lag, alertness, tenseness, and daytime fatigue were assessed using visual analog scales (VAS) and standardized questionnaires. The objective markers of readaptation included core body temperature, wrist actigraphy, cortisol and electrolyte excretion, and a battery of computerized performance tests.

Results: HD but not LD enhanced the readaptation speed of all physiological rhythms investigated, as demonstrated by a significantly faster movement of acrophases towards the post-shift target time. HD (p=0.05) significantly blunted the post-shift deterioration of performance in those tests that were sensitive to shift lag. Parameters of subjective well-being were not significantly affected by either dose.

Conclusion: This pilot study demonstrates the chronobiotic efficacy of LY when taken at a dose of 5 mg/d.     

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.     

Chronobiologic perspectives of black time—Accident risk is greatest at night: An opinion paper

Simon Folkard in 1997 introduced the phrase black time to draw attention to the fact that the risk of driving accidents (DA) is greater during the night than day in usually diurnally active persons. The 24 h temporal pattern in DA entails circadian rhythms of fatigue and sleep propensity, cognitive and physical performance, and behavior that are controlled, at least in part, by endogenous clocks. This opinion paper extends the concept of black time to reports of excess nighttime accidents and injuries of workers and nocturnal occurrence of certain man-caused catastrophes. We explore the chronobiology of work-related black time accidents and injuries taking into account laboratory and field investigations describing, respectively, circadian rhythms in cognitive performance and errors and mistakes by employees in the conduct of routine occupational tasks. Additionally, we present results of studies pertaining to 24 h patterns of both the number and relative risk (number of events per h/number of workers exposed per h) of work-related accidents (WRA) and injuries (WRI) as well as indices of performance and alertness of a self-selected homogenous survivor cohort of French firefighters (FFs) to explore two possible explanations of black time, namely, 24 h variation in sleep propensity/drossiness characterized by a nocturnal peak and circadian rhythms in cognitive performance characterized by a nocturnal trough. We propose the 24 h pattern of WRA and WRI, particularly of FFs and other highly skilled self-selected cohorts, is more strongly linked to circadian rhythms of fatigue and sleepiness than cognitive performance. Other possible explanations –suppressed expression of circadian rhythms and/or unmasking of ultradian periodicities in cognitive performance in specific circumstances, e.g., highly stressful work, competitive, or life-threatening settings, are also discussed.     

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.     

Circadian determinants of subjective alertness

Four healthy male subjects each experienced a temporal isolation experiment lasting several months. During part of each experiment (2-5 weeks), the subject's entire imposed daily routine (including light-dark, rest-activity, and meal routines) was either stretched (two subjects: T = 25.8 hr, 26.0 hr) or reduced (two subjects: T = 22.8 hr, 23.1 hr) to beyond the range of entrainment of the endogenous circadian pacemaker (ECP), which then ran at a different period (tau). Subjective alertness was measured approximately three times per hour (during wakefulness), using a computerized visual analogue scale technique. Circadian rhythms in subjective alertness were then plotted both at tau, the period length of the ECP, and at T, the period length of the imposed sleep-wake cycle (SWC) and light-dark cycle. At tau, the alertness rhythm was closely in phase with the temperature rhythm. At T, the alertness rhythm showed an "inverted-U" function with a peak toward the middle of the subjective day, upon which was superimposed a "postlunch dip" for one subject. Thus, subjective alertness would appear to be under the control fo both ECP and SWC mechanisms, which combine to produce the composite time-of-day function normally observed in a diurnal setting.     

Evaluation of a fast forward rotating shift schedule in the steel industry with a special focus on ageing and sleep

The aim of this study was to explore the effect of a change in the speed and direction of shift rotation on the sleep and wakefulness of younger and older workers. A continuous three-shift schedule was changed from a slow backward rotating (EEE-MMMNNN-----) to a fast forward rotating system (MMEENN----). Sixteen subjects (mean age 42 years) were studied before and one year after the change in schedule. Two age groups were compared: ten younger men (mean age 35 years) and six older men (mean age 53 years). The effects of the new work schedule were evaluated by a questionnaire (modified SSI), and on-site registrations with an actigraph and sleep log for one shift cycle (10-15 days) before and after the new schedule. After the change in schedule, subjective sleep problems decreased and alertness increased during the morning shifts. The change in schedule influenced sleep differently in the two age-groups. Both the subjective and objective quality of sleep improved among the older workers. The results indicate that a fast forward rotating shift schedule is more suitable for older workers than a slower backward rotating system.     

Acute effects of different light spectra on simulated night-shift work without circadian alignment

Short-wavelength and short-wavelength-enhanced light have a strong impact on night-time working performance, subjective feelings of alertness and circadian physiology. In the present study, we investigated acute effects of white light sources with varied reduced portions of short wavelengths on cognitive and visual performance, mood and cardiac output.Thirty-one healthy subjects were investigated in a balanced cross-over design under three light spectra in a simulated night-shift paradigm without circadian adaptation.Exposure to the light spectrum with the largest attenuation of short wavelengths reduced heart rate and increased vagal cardiac parameters during the night compared to the other two light spectra without deleterious effects on sustained attention, working memory and subjective alertness. In addition, colour discrimination capability was significantly decreased under this light source.To our knowledge, the present study for the first time demonstrates that polychromatic white light with reduced short wavelengths, fulfilling current lighting standards for indoor illumination, may have a positive impact on cardiac physiology of night-shift workers without detrimental consequences for cognitive performance and alertness.     

Circadian rhythms and their association with body temperature and time awake when performing a simple task with the dominant and non-dominant hand

The two main aims of the study were to compare the dominant and non-dominant hand with regard to circadian rhythms of accuracy of performance at a task that required eye-hand coordination and sub-maximum muscle contraction, as well as to investigate if there were differences between the dominant and non-dominant hands in the associations between circadian rhythms of performance and core temperature and time awake. The task consisted of using a larger counter to flick a set of 20 smaller counters to land as near as possible to the center of a target. The nearer to the center of the target a counter landed, the higher the score awarded. Three measures of accuracy were calculated: the total score, the number of times the counter missed the target altogether, and the mean score for those counters that hit the target. Seventy-eight healthy participants performed the task at each of six test sessions distributed every 4 h throughout the day (at 08:00, 12:00... . 04:00 h), the participants then having been awake for about 1, 4... . 20 h, respectively. Before each test session, sub-lingual temperature (an estimate of core temperature) was measured, and estimates of the individual's alertness and fatigue were obtained. Temperature, alertness, and fatigue all showed circadian rhythms that were phased conventionally. Measures of accuracy of performance also showed significant circadian rhythms that were phased closer to the rhythms of alertness and fatigue than to that of oral temperature. In addition, and in support of our previous work, there were significant associations between performance and temperature (positive) and time awake (negative) for most measures of accuracy. Even though circadian rhythms of performance accuracy and effects of oral temperature and time awake were generally very similar between the dominant and non-dominant hand, there was a suggestion that time awake affected some aspects of performance of the non-dominant hand to a greater extent. There was little evidence to support the view that the 24-h rhythmicity was less marked in the non-dominant hand, which argues against internal desynchronization, at least for the task used in this study.     

Relationship between alertness,performance, and body temperature in humans

Body temperature has been reported to influence human performance. Performance is reported to be better when body temperature is high/near its circadian peak and worse when body temperature is low/near its circadian minimum. We assessed whether this relationship between performance and body temperature reflects the regulation of both the internal biological timekeeping system and/or the influence of body temperature on performance independent of circadian phase. Fourteen subjects participated in a forced desynchrony protocol allowing assessment of the relationship between body temperature and performance while controlling for circadian phase and hours awake. Most neurobehavioral measures varied as a function of internal biological time and duration of wakefulness. A number of performance measures were better when body temperature was elevated, including working memory, subjective alertness, visual attention, and the slowest 10% of reaction times. These findings demonstrate that an increased body temperature, associated with and independent of internal biological time, is correlated with improved performance and alertness. These results support the hypothesis that body temperature modulates neurobehavioral function in humans.     

Relationships between the circadian rhythms of finger temperature, core temperature, sleep latency, and subjective sleepiness

Skin temperature circadian rhythms have been explored relatively recently. It has been suggested that distal and proximal skin temperature changes play a role in the regulation of the core temperature circadian rhythm and sleepiness. The authors investigated the circadian finger and core temperature rhythms in conjunction with the circadian rhythms of subjective and objective sleepiness. Fourteen healthy, young, good sleepers participated in a modified constant-routine procedure in which palmar finger temperature, rectal temperature, subjective sleepiness, and objective sleep latency were measured half-hourly across a 48-h period of enforced wakeful bed rest. Individual curves were adjusted to the group mean temperature minimum time of 0500 h and averaged to create the 4 mean curves. The 5 possible cross-correlation curves between these 4 measures were calculated for half-hourly phase lags from 12 h before to 12 h after the group mean core temperature minimum time. Maximum cross-correlations for each curve suggested that finger temperature preceded core temperature by 3 h (r = -0.22), and subjective sleepiness followed core temperature by 0.5 h (r = -0.33) and objective sleepiness by 2 h (r = 0.29). Although these data are correlational, they are consistent with the notion that finger temperature changes drive core temperature changes, which determine changes of subjective and objective sleepiness.     

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.     

Heritability of morningness-eveningness and self-report sleep measures in a family-based sample of 521 hutterites

Individual variation in the phase and amplitude of human circadian rhythms is well known, but the impact of heritable factors on such variation is less clear. We estimated the narrow-sense heritability for selected circadian and sleep timing, quality, and duration measures among related members of the Hutterites, an endogamous, religious community (n=521 participants). “Morningness-eveningness” (M/E), a stable trait reflecting circadian phase, was evaluated using the Composite Scale (CS). Subjective sleep measures were assessed using the Sleep Timing Questionnaire. Initial analyses reconfirmed the impact of age on M/E. Previously reported correlations between M/E scores and the sleep measures were also noted, demonstrating the construct validity of the questionnaires among the participants. Following corrections for age, gender, and colony of residence, significant narrow-sense heritability was noted for M/E (23%). The heritability for subjective sleep measures (related to timing, duration, and quality) were statistically significant for all but one variable, and varied between 12.4% and 29.4%. Thus, significant heritable influences on human circadian phase and subjective sleep indices can be detected through family-based studies. In view of the impact of circadian malfunction on human health, it may be worthwhile to map genetic factors impacting circadian and sleep variation.     

The Tick-Tock of Language: Is Language Processing Sensitive to Circadian Rhythmicity and Elevated Sleep Pressure?

The master circadian pacemaker emits signals that trigger organ-specific oscillators and, therefore, constitutes a basic biological process that enables organisms to anticipate daily environmental changes by adjusting behavior, physiology, and gene regulation. Although circadian rhythms are well characterized on a physiological level, little is known about circadian modulations of higher cognitive functions. Thus, we investigated circadian repercussions on language performance at the level of minimal syntactic processing by means of German noun phrases in ten young healthy men under the unmasking conditions of a 40 h constant-routine protocol. Language performance for both congruent and incongruent noun phrases displayed a clear diurnal rhythm with a peak performance decrement during the biological night. The nadirs, however, differed such that worst syntactic processing of incongruent noun phrases occurred 3 h earlier (07:00 h) than that of congruent noun phrases (10:00 h). Our results indicate that language performance displays an internally generated circadian rhythmicity with optimal time for parsing language between 3 to 6 h after the habitual wake time, which usually corresponds to 10:00–13:00 h. These results may have important ramifications for establishing optimal times for shiftwork changes or testing linguistically impaired people.     

The influence of subjective alertness and motivation on human performance independent of circadian and homeostatic regulation

Endogenous circadian rhythmicity and sleep-wake homeostasis are robust regulators of human alertness and performance, yet few studies have examined how these regulatory processes affect motivation. Moreover, the influence of alertness and motivation on performance, independent of circadian phase and hours awake, has not been studied. Healthy subjects, 12 males and 3 females, ages 20 to 41, participated in a 2-week 28-h forced desynchrony protocol to address these issues. Subjects performed a battery of tests every 2 hours during scheduled wakefulness. Performance on a mathematical addition test and ratings of alertness and motivation on visual analog scales were analyzed. Performance scores were categorized as being associated with the highest or lowest alertness and motivation ratings for each circadian phase/hours awake bin to determine whether high levels of alertness and motivation resulted in higher performance scores above and beyond the effects of circadian and homeostatic regulation. Motivation varied significantly as a function of circadian phase and hours awake. Motivation and alertness were correlated. When circadian phase and hours awake were accounted for, performance was better when alertness and motivation ratings were highest and worse when those ratings were lowest. The present findings suggest that human performance is influenced by alertness and motivation independent of circadian phase and hours awake. Future studies examining the influence of circadian phase and sleep-wake homeostasis on human performance also should assess alertness and motivation to aid in the interpretation of performance data. Such studies also may aid in the development of countermeasures to improve human performance.     

Alertness and psychomotor performance levels of marine pilots on an irregular work roster

ABSTRACT

Fatigue is recognized as an important safety concern in the transportation industry. In this study, our goal was to investigate how circadian and sleep–wake dependent factors influence St-Lawrence River pilots’ sleep–wake cycle, alertness and psychomotor performance levels at work. A total of 18 male St-Lawrence River ship pilots were recruited to a 16–21-day field study. Pilots’ chronotype, sleepiness and insomnia levels were documented using standardized questionnaires. Their sleep–wake cycle was documented by a sleep–wake log and wrist-worn activity monitoring. Subjective alertness and objective psychomotor performances were assessed ~5×/day for each work and rest day. Ship transits were distributed throughout the 24-h day and lasted on average (± SEM) 5.93 ± 0.67 h. Main sleep periods occurred mainly at night, and objectively lasted 6.04 ± 1.02 h before work days. When going to bed at the end of work days, pilots subjectively reported sleeping 7.64 ± 1.64 h in the prior 24 h. Significant diurnal and wake-dependent effects were observed for subjective alertness and objective psychomotor performance, with minimum levels occurring between 09:00 and 10:00. Thus, despite their irregular work schedule, ship pilots presented, as a group, a diurnal variation of alertness and psychomotor performance indicative of a day-oriented circadian system. Important inter-individual differences were observed on psychomotor performance mesor and phase. In individuals, earlier phases in psychomotor performance were correlated with earlier chronotype. This study indicates that both circadian and homeostatic processes modulate alertness and psychomotor performance levels with worst levels reached when long shifts ended in the morning. This work has potential applications as it indicates fatigue countermeasures considering both processes are scientifically based.