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.     

On mathematical modeling of circadian rhythms, performance, and alertness

Mathematical models of neurobehavioral performance and alertness have both basic science and practical applications. These models can be especially useful in predicting the effect of different sleep-wake schedules on human neurobehavioral objective performance and subjective alertness under many conditions. Several relevant models currently exist in the literature. In principle, the development and refinement of any mathematical model should be based on an explicit modeling methodology, such as the Box modeling paradigm, that formally defines the model structure and calculates the set of parameters. While most mathematical models of neurobehavioral performance and alertness include homeostatic, circadian, and sleep inertia components and their interactions, there may be fundamental differences in the equations included in these models. In part, these may be due to differences in the assumptions of the underlying physiology. Because the choice of model equations can have a dramatic influence on the results, it is necessary to consider these differences in assumptions when examining the results from a model and when comparing results across models. This article presents principles of mathematical modeling and examples of how such procedures can be applied to the development and refinement of mathematical models of neurobehavioral performance and alertness. This article also presents several methods of testing and comparing these models, suggests different uses of the models, and discusses problems with current models.     

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.     

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

Melatonin: characteristics, concerns, and prospects

Melatonin is of great importance to the investigation of human biological rhythms. Its rhythm in plasma or saliva provides the best available measure of the timing of the internal circadian clock. Its major metabolite 6-sulphatoxymelatonin is robust and easily measured in urine. It thus enables long-term monitoring of human rhythms in real-life situations where rhythms may be disturbed, and in clinical situations where invasive procedures are difficult. Melatonin is not only a "hand of the clock"; endogenous melatonin acts to reinforce the functioning of the human circadian system, probably in many ways. Most is known about its relationship to sleep and the decline in core body temperature and alertness at night. Current perspectives also include a possible influence on major disease risk, arising from circadian rhythm disruption. Melatonin clearly has the ability to induce sleepiness and lower core body temperature during "biological day" and to change the timing of human rhythms when treatment is appropriately timed. It can entrain free-running rhythms and maintain entrainment in most blind and some sighted people. Used therapeutically it has proved a successful treatment for circadian rhythm disorder, particularly the non-24-h sleep wake disorder of the blind. Numerous other clinical applications are under investigation. There are, however, areas of controversy, large gaps in knowledge, and insufficient standardization of experimental conditions and analysis for general conclusions to be drawn with regard to most situations. The future holds much promise for melatonin as a therapeutic treatment. Most interesting, however, will be the dissection of its effects on human genes.     

Characterizing the amplitude dynamics of the human core-temperature circadian rhythm using a stochastic-dynamic model

Two measures, amplitude and phase, have been used to describe the characteristics of the endogenous human circadian pacemaker, a biological clock located in the hypothalamus. Although many studies of change in circadian phase with respect to different stimuli have been conducted, the physiologic implications of the amplitude changes (dynamics) of the pacemaker are unknown. It is known that phase changes of the human circadian pacemaker have a significant impact on sleep timing and content, hormone secretion, subjective alertness and neurobehavioral performance. However, the changes in circadian amplitude with respect to different stimuli are less well documented. Although amplitude dynamics of the human circadian pacemaker are observed in physiological rhythms such as plasma cortisol, plasma melatonin and core temperature data, currently methods are not available to accurately characterize the amplitude dynamics from these rhythms. Of the three rhythms core temperature is the only reliable variable that can be monitored continuously in real time with a high sampling rate. To characterize the amplitude dynamics of the circadian pacemaker we propose a stochastic-dynamic model of core temperature data that contains both stochastic and dynamic characteristics. In this model the circadian component that has a dynamic characteristic is represented as a perturbation solution of the van der Pol equation and the thermoregulatory response in the data that has a stochastic characteristic is represented as a first-order autoregressive process. The model parameters are estimated using data with a maximum likelihood procedure and the goodness-of-fit measures along with the associated standard error of the estimated parameters provided inference about the amplitude dynamics of the pacemaker. Using this model we analysed core temperature data from an experiment designed to exhibit amplitude dynamics. We found that the circadian pacemaker recovers slowly to an equilibrium level following amplitude suppression. In humans this reaction to perturbation from equilibrium value has potential physiological implications.     

Differential 24‐Hour Variation of Alertness and Subjective Tension in Process Controllers: Investigation of the Relationship with Body Temperature and Heart Rate

The effects of shift and time‐on‐shift on alertness and perceived tension, as well as related physiological variables, were investigated in satellite controllers working a rapid forward rotating three‐shift system. In controlled laboratory conditions, subjective tension and HR have been reported to display circadian variation and marked sensitivity to external factors. We examined whether circadian variations were masked for these particular variables in real‐job conditions, unlike for alertness and body temperature, which have been repeatedly shown to display circadian variation in these conditions. This hypothesis was tested in a repeated‐measures design by collecting alertness and tension self‐reports and recording operators' sublingual temperature on three occasions on each shift and HR continuously throughout shifts. Alertness and body temperature varied according to a typical diurnal trend; subjective tension was only enhanced on the initial recording of each shift (compared to the remaining ones), while HR displayed an intermediary trend. Intra‐subject correlations revealed a positive relationship between alertness, oral temperature, and HR, while no such relationship was found for subjective tension. These results support the hypothesis of a close dependence of alertness and temperature, and to a lesser extent for HR, on endogenous mechanisms in this job‐situation. In addition, some situation‐specific factors, such as job‐demand, would affect subjective tension and partially mask the circadian variations in HR.     

Intrinsic period and light intensity determine the phase relationship between melatonin and sleep in humans

The internal circadian clock and sleep-wake homeostasis regulate the timing of human brain function, physiology, and behavior so that wakefulness and its associated functions are optimal during the solar day and that sleep and its related functions are optimal at night. The maintenance of a normal phase relationship between the internal circadian clock, sleep-wake homeostasis, and the light-dark cycle is crucial for optimal neurobehavioral and physiological function. Here, the authors show that the phase relationship between these factors-the phase angle of entrainment (psi)-is strongly determined by the intrinsic period (tau) of the master circadian clock and the strength of the circadian synchronizer. Melatonin was used as a marker of internal biological time, and circadian period was estimated during a forced desynchrony protocol. The authors observed relationships between the phase angle of entrainment and intrinsic period after exposure to scheduled habitual wakefulness-sleep light-dark cycle conditions inside and outside of the laboratory. Individuals with shorter circadian periods initiated sleep and awakened at a later biological time than did individuals with longer circadian periods. The authors also observed that light exposure history influenced the phase angle of entrainment such that phase angle was shorter following exposure to a moderate bright light (approximately 450 lux)-dark/wakefulness-sleep schedule for 5 days than exposure to the equivalent of an indoor daytime light (approximately 150 lux)-dark/wakefulness-sleep schedule for 2 days. These findings demonstrate that neurobiological and environmental factors interact to regulate the phase angle of entrainment in humans. This finding has important implications for understanding physiological organization by the brain's master circadian clock and may have implications for understanding mechanisms underlying circadian sleep disorders.     

Differential 24-hour variation of alertness and subjective tension in process controllers: investigation of the relationship with body temperature and heart rate

The effects of shift and time-on-shift on alertness and perceived tension, as well as related physiological variables, were investigated in satellite controllers working a rapid forward rotating three-shift system. In controlled laboratory conditions, subjective tension and HR have been reported to display circadian variation and marked sensitivity to external factors. We examined whether circadian variations were masked for these particular variables in real-job conditions, unlike for alertness and body temperature, which have been repeatedly shown to display circadian variation in these conditions. This hypothesis was tested in a repeated-measures design by collecting alertness and tension self-reports and recording operators' sublingual temperature on three occasions on each shift and HR continuously throughout shifts. Alertness and body temperature varied according to a typical diurnal trend; subjective tension was only enhanced on the initial recording of each shift (compared to the remaining ones), while HR displayed an intermediary trend. Intra-subject correlations revealed a positive relationship between alertness, oral temperature, and HR, while no such relationship was found for subjective tension. These results support the hypothesis of a close dependence of alertness and temperature, and to a lesser extent for HR, on endogenous mechanisms in this job-situation. In addition, some situation-specific factors, such as job-demand, would affect subjective tension and partially mask the circadian variations in HR.     

Sleep, performance, circadian rhythms, and light-dark cycles during two space shuttle flights

Sleep, circadian rhythm, and neurobehavioral performance measures were obtained in five astronauts before, during, and after 16-day or 10-day space missions. In space, scheduled rest-activity cycles were 20-35 min shorter than 24 h. Light-dark cycles were highly variable on the flight deck, and daytime illuminances in other compartments of the spacecraft were very low (5.0-79.4 lx). In space, the amplitude of the body temperature rhythm was reduced and the circadian rhythm of urinary cortisol appeared misaligned relative to the imposed non-24-h sleep-wake schedule. Neurobehavioral performance decrements were observed. Sleep duration, assessed by questionnaires and actigraphy, was only approximately 6.5 h/day. Subjective sleep quality diminished. Polysomnography revealed more wakefulness and less slow-wave sleep during the final third of sleep episodes. Administration of melatonin (0.3 mg) on alternate nights did not improve sleep. After return to earth, rapid eye movement (REM) sleep was markedly increased. Crewmembers on these flights experienced circadian rhythm disturbances, sleep loss, decrements in neurobehavioral performance, and postflight changes in REM sleep.     

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)     

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.     

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.     

THE INFLUENCE OF CIRCADIAN PHASE AND PRIOR WAKE ON NEUROMUSCULAR FUNCTION

Previous forced desynchrony (FD) studies have shown that neurobehavioral function is affected by circadian phase and duration of prior wakefulness. There is some evidence that neuromuscular function may also be affected by circadian phase and prior wake, but these effects have not been systematically investigated. This study examined the effects of circadian phase and prior wake on two measures of neuromuscular function—postural balance (PB) and maximal grip strength (MGS)—using a 28-h FD protocol. Eleven male participants (mean?±?SD: 22.7?±?2.5 yr) lived in a sound-attenuated, light- and temperature-controlled time-isolation laboratory for 12 days. Following two training days and a baseline day, participants were scheduled to seven 28-h FD days, with the ratio between sleep opportunity and wake spans kept constant (i.e., 9.3?h sleep period and 18.7?h wake period). PB was measured during 1?min of quiet standing on a force platform. MGS of the dominant hand was measured using a dynamometer. These two measures were obtained every 2.5?h during wake. Core body temperature was continuously recorded with rectal thermistors to determine circadian phase. For both measures of neuromuscular function, individual data points were assigned a circadian phase and a level of prior wake. Data were analyzed by repeated-measures analysis of variance (ANOVA) with two within-subjects factors: circadian phase (six phases) and prior wake (seven levels). For MGS, there was a main effect of circadian phase, but no main effect of prior wake. For PB, there were no main effects of circadian phase or prior wake. There were no interactions between circadian phase and prior wake for MGS or PB. The significant effect of circadian phase on muscle strength is in agreement with previous reports in the literature. In terms of prior wake, both MGS and PB remained relatively stable across wake periods, indicating that neuromuscular function may be more robust than neurobehavioral function when the duration of wakefulness is within a normal range (i.e., 18.7?h). (Author correspondence: charli.sargent@unisa.edu.au)     

Circadian Rhythm of Blood Ethanol Clearance Rates in Rats: Response to Reversal of the L/D Regimen and to Continuous Darkness and Continuous Illumination

Phase relationships of the circadian rhythms of blood ethanol clearance (metabolic) rates and body temperature were studied in rats successively exposed to 4 illumination regimens: LD (light from 0800-2000 hr), DL (light from 2000-0800 hr), constant darkness (DD) and, lastly, constant light (LL). After a 4-wk standardization to each regimen, body temperatures were taken at 9 × 4-hr intervals to establish baseline circadian profiles. One week later, groups (N = 8) received 1.5 g/kg ethanol (i.p.) at 6 equally spaced timepoints during a 24-hr span, when temperatures were again measured. Ethanol clearance rates were estimated from decreasing blood ethanol levels sampled every 20 min from 60-200 min after dosing, and the resultant elimination curves were subjected to cosinor analysis. These studies show for the first time that the high amplitude circadian rhythm in ethanol metabolism persists under constant conditions of illumination (DD and LL), demonstrating that it may well be a truly internal circadian rhythm and not a response to exogenous cues of the light/dark cycle. During both LD and DL, maximal and minimal ethanol clearance rates fell near the end of the dark and light phases, respectively, and followed circadian peak and trough control temperatures by approximately 6 hr. A fixed internal phase relationship between the core body temperature and the circadian rhythm in ethanol metabolism is demonstrated, thus establishing the rhythm in body temperature as a suitable and convenient internal marker rhythm for studies of the metabolism of low-to-moderate ethanol doses. These studies demonstrate that the phase relationships of blood ethanol clearance rate and body temperature can be manipulated by the illumination regimen selected, an observation of both basic and practical importance.     

Comparison of Amplitude Recovery Dynamics of Two Limit Cycle Oscillator Models of the Human Circadian Pacemaker

At an organism level, the mammalian circadian pacemaker is a two‐dimensional system. For these two dimensions, phase (relative timing) and amplitude of the circadian pacemaker are commonly used. Both the phase and the amplitude (A) of the human circadian pacemaker can be observed within multiple physiological measures—including plasma cortisol, plasma melatonin, and core body temperature (CBT)—all of which are also used as markers of the circadian system. Although most previous work has concentrated on changes in phase of the circadian system, critically timed light exposure can significantly reduce the amplitude of the pacemaker. The rate at which the amplitude recovers to its equilibrium level after reduction can have physiological significance. Two mathematical models that describe the phase and amplitude dynamics of the pacemaker have been reported. These models are essentially equivalent in predictions of phase and in predictions of amplitude recovery for small changes from an equilibrium value (A=1), but are markedly different in the prediction of recovery rates when A<0.6. To determine which dynamic model best describes the amplitude recovery observed in experimental data; both models were fit to CBT data using a maximum likelihood procedure and compared using Akaike's Information Criterion (AIC). For all subjects, the model with the lower recovery rate provided a better fit to data in terms of AIC, supporting evidence that the amplitude recovery of the endogenous pacemaker is slow at low amplitudes. Experiments derived from model predictions are proposed to test the influence of low amplitude recovery on the physiological and neurobehavioral functions.     

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 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)     

The effect of a change in sleep-wakefulness timing, bright light and physical exercise interventions on 24-hour patterns of performance, mood and body temperature

Experiments consisting of baseline, bright light and physical exercise studies were carried out to compare the effect of a 9-hour delay in sleep-wakefulness timing, and the effects of bright light and physical exercise interventions on 24-hour patterns of performance, mood and body temperature were examined. Each study comprised a 24-hour constant routine at the beginning followed by 3 night shifts and 24-hour constant routine at the end. Performance on tasks differing in cognitive load, mood and body temperature was measured during each constant routine and the interventions were applied during the night shifts. The 24-hour pattern of alertness and performance on the tasks with low cognitive load in post-treatment conditions followed the change in sleep-wakefulness timing while more cognitively loaded tasks tended to show a reverse trend when compared to pre-treatment conditions. There was a phase delay around 4 hours in circadian rhythms of body temperature in post-treatment conditions.