A circadian oscillator model based on empirical data.

A model based on the van der Pol equation has been developed to predict the pattern of adaptation of aircrew and other travellers to rapid time-zone transitions, when the exposure to light cannot be quantified. The parameters of the model include the stiffness (mu) and the intrinsic period (T0), which together define the free-running period, and the external force (F). The parameter values were estimated by using a simplex minimization technique to fit the output from the model to body temperature data from 12 individuals before, and over a 12-day period immediately after, a 10-h eastward transition between London and Sydney. Data were collected at three equally spaced points during each sleep period and at the end of four 45-min rest periods during the day. The fitting procedure enabled the parameters of the temperature rhythm to be estimated after correcting for the masking effect of sleep. The average estimates of mu (0.38 h) and T0 (24.24 h) were close to earlier estimates based on forced desynchronization experiments, and the mean free-running period, calculated from these, was 24.50 h. The mean value of the external force F (0.54) was surprisingly high, and this may reflect the strong outdoor light levels during the days in Sydney. Estimates of phase, based on the model solutions, suggested that 11 subjects adapted by a phase delay and 1 by a phase advance. However, the amplitude of the rhythms was much reduced at times when the phase was changing rapidly. Simulations using the range of the model parameters for the 12 individuals predicted that adaptation to within 1 h after a 10-h eastward transition would be achieved within between 3 and 11 days. However, since these predictions are dependent on the choice of external force, estimates may need to be more conservative in real-life situations when light exposure cannot be measured.

Beyond the three-process model of alertness: estimating phase, time on shift, and successive night effects.

This paper starts by summarizing the development and refinement of the additive three-process model of alertness first published by Folkard and Akerstedt in 1987. It reviews some of the successes that have been achieved by the model in not only predicting variations in subjective alertness on abnormal sleep-wake schedules but also in accounting for objective measures of sleep latency and duration. Nevertheless, predictions derived from the model concerning alertness on different shifts, and over successive night shifts, are difficult to reconcile with published data on accident risk. In light of this, we have examined two large sets of alertness ratings with a view to further refining the model and identifying additional factors that may influence alertness at any given point in time. Our results indicate that, at least for the range of sleep durations and wake-up times commonly found on rotating shift systems, we may assume the phase of the endogenous circadian component of alertness (process C) to be "set" by the time of waking. Such an assumption considerably enhanced the predictive power of the model and yielded remarkably similar phase estimates to those obtained by maximizing the post-hoc fit of the model. We then examined the manner in which obtained ratings differed from predicted values over a complete 8-day cycle of two, 12-h shift systems. This revealed a pronounced "first night compensation effect" that resulted in shift workers rating themselves as progressively more alert than would be predicted over the course of the first night shift. However, this appeared to be achieved only at the cost of lowered ratings on the second night shift. Finally, we were able to identify a "time on shift" effect whereby, with the exception of the first night shift, alertness ratings decreased over the course of each shift before showing a modest "end effect." We conclude that the identification of these additional components offers the possibility that in the future we may be able to predict trends in accident risk on abnormal sleep-wake schedules.

A phenomenological variant of ecological systems theory (PVEST): a self-organization perspective in context.

A framework that emphasizes and integrates individuals' intersubjective experiences with Bronfenbrenner's ecological systems theory (PVEST) is introduced and compared with self-organizational perspectives. Similarities, differences and advantages of each framework are described. In a demonstration of PVEST's utility, a subset of data from the 3rd year of a longitudinal study (14- to 16-year-old middle adolescent African-Americans) is used for examining an achievement variable: negative learning attitude. Explored separately by gender, a regression model that contained risk, stress, and a reactive coping variable for the prediction of negative learning attitudes was investigated. For boys, stress was an independent stressor across steps independent of the other variables entered; social support variable, perceived unpopularity with peers, that was a significant predictor of girls' negative learning attitude. Particularly for boys, the findings suggest critically important roles for teachers and peers in the negative learning attitude of midadolescent economically disadvantaged African-American students.

Educational risk and resilience in African-American youth: context, self, action, and outcomes in school.

This study examined the empirical validity of a model of human motivation as it applies to school success and failure in 3 independent samples of 10- to 16-year-old African-American youth. Specifically, we assessed how indicators of context, self, and action relate to measures of risk and resilient outcomes in school in 3 different samples, using 3 different measurement strategies. Correlational and path analyses on the 3 data sets supported the empirical validity of the model. African-American youth's experience of their parents' school involvement predicted a composite of self-system processes, which in turn predicted the subjects' reports of their engagement in school. Engagement then predicted school performance and adjustment. The data supported a reciprocal path from action to context, suggesting that youth who show more disaffected patterns of behavior and emotion in school experience less support from their families than those reporting more engaged patterns of action. Implications for program and policy decisions are discussed.

Influence of polar route schedules on the duty and rest patterns of aircrew.

The duty and rest periods of aircrew operating the polar route from London via Anchorage were recorded during five schedules which involved 1-, 2- or 3-day sojourns in Japan. Sleep throughout each schedule was fragmented, with naps before duty and short sleeps after arrival at a new location. Sleep disturbance rather than cumulative sleep loss appeared to be the overriding problem, and the shorter schedules had the most marked disturbances in sleep during the trip and during the immediate recovery period. Electroencephalographic studies are necessary to confirm these observations, and information on circadian rhythmicity is needed to define the circumstances which lead to persistence of sleep disturbance on return to Europe.

Jet lag and motion sickness.

Jet lag. Present day aircraft operating round northern and southern latitudes cross time zones at almost the same rate as the earth rotates, and it is these rapid transmeridian transitions that lead to the syndrome commonly referred to as jet lag. On arrival at their destination, individuals find themselves out of synchrony with the social and time cues of their new environment and, until they adapt, may experience symptoms such as malaise, gastrointestinal disturbance, loss of appetite, tiredness during the day and poor sleep. The severity and exact nature of the problems vary with the direction of travel and the number of time zones crossed, and some people react more unfavourably to intercontinental travel than others. Clearly, with increasing numbers of passengers undertaking such journeys, there is considerable interest in strategies to reduce the immediate effects of jet lag or to facilitate acclimatisation. Motion sickness is a generic term which embraces seasickness, airsickness, carsickness, space sickness etc, names that identify the provocative environment or vehicle. It is a normal reaction of humans to exposure to certain motion stimuli that occur during passive transportation.

A mathematical model of the human circadian system and its application to jet lag.

A mathematical model of the circadian system is described that is appropriate for application to jet lag. The core of the model is a van der Pol equation with an external force. Approximate solutions of this equation in which the external force is composed of a constant and an oscillating term are investigated. They lead to analytical expressions for the amplitude and period of free-running rhythms and for the frequency limits of the entrainment region. The free-running period increases quadratically with stiffness. Both period and amplitude depend on the value of the constant external force. The width of the range of entrainment is mostly determined by the external force, whereas the relative position of this range follows the intrinsic period of the oscillator. Experiments with forced and spontaneous internal desynchronization were evaluated using these analytical expressions, and estimates were obtained for the intrinsic period of the oscillator, its stiffness, and the external force. A knowledge of these model parameters is essential for predictions about circadian dynamics, and there are practical implications for the assessment of the adaptation after rapid trans-meridian travel.

Circadian rhythmicity and sleep of aircrew during polar schedules.

Sleep and circadian rhythms of aircrew were studied during a 7-d polar schedule operated between London and Tokyo. Sleep, rectal temperature, and subjective alertness were recorded for 2 d before departure during the schedule, and for 10 d after the return. Changes in sleep during the early part of the trip were due to sleep loss on the outward journey, but later these changes were related to the displacement of the circadian rhythm. The acrophases of the circadian rhythms of temperature were delayed by the outward journey, and amplitudes were reduced throughout the trip. During the return, aircrew reported high levels of tiredness which persisted until the second recovery night. Though the amounts of sleep obtained during the schedule were satisfactory for the aircrew as a group, some crewmembers experienced difficulties. Realignment of circadian rhythms was attained by an advance of the circadian phase in eight aircrew and by a delay in three, and resynchronization was achieved in all cases within 6 d.

Performance and quality of sleep wearing NBC protective clothing.

Two separate studies were carried out to investigate the effect of wearing nuclear-biological-chemical aircrew equipment assembly (NBC AEA) protective clothing on performance and on overnight sleep. Performance at a series of tasks was measured, in six male subjects, during the day (0800-2000 hours) and at night (2000-0800 hours). Wearing the NBC assembly did not lead to significant decrements in performance compared with the normal aircrew equipment assembly (AEA). The sleep of six male volunteers was recorded electroencephalographically on two consecutive nights when NBC protective clothing was worn. Sleep was both shortened and disturbed, compared with overnight control sleep. There were some improvements on the second night, suggesting that individuals may adapt to wearing the NBC assembly.

The influence of a 1 h nap on performance overnight.

The effect on performance overnight of a 1 h nap taken at 0200 h was studied in six young female subjects. The subjects completed three schedules, including one with a nap and two without a nap, during which either a placebo or 300 mg caffeine was ingested at 2315 h. Performance was measured from 1700 h in the evening until 1030 h the next morning. Caffeine improved performance overnight on almost all tasks compared with placebo. The nap had some limited beneficial effect compared with placebo, but most tasks remained impaired.

Regression models for the estimation of circadian rhythms in the presence of effects due to masking.

The estimation of human circadian rhythms from experimental data is complicated by the presence of "masking" effects associated with the sleep-wake cycle. The observed rhythm may include a component due to masking, as well as the endogenous component linked to a circadian pacemaker. In situations where the relationship between the sleep-wake cycle and the circadian rhythm is not constant, it may be possible to obtain individual estimates of these two components, but methods commonly used for the estimation of circadian rhythms, such as the cosinor analysis, spectral analysis, average waveforms and complex demodulation, have not generally been adapted to identify the modulations that arise from masking. The estimates relate to the observed rhythms, and the amplitudes and acrophases do not necessarily refer to the endogenous rhythm. In this paper methods are discussed for the separation of circadian and masking effects using regression models that incorporate a sinusoidal circadian variation together with functions of time since sleep and time during sleep. The basic model can be extended to include a time-varying circadian rhythm and estimates are available for the amplitude and phase at a given time, together with their joint confidence intervals and tests for changes in amplitude and acrophase between any two selected times. Modifications of these procedures are discussed to allow for non-sinusoidal circadian rhythms, non-additivity of the circadian and time-since-sleep effects and the breakdown of the usual assumptions concerning the residual errors. This approach enables systematic masking effects associated with the sleep-wake cycle to be separated from the circadian rhythm, and it has applications to the analysis of data from experiments where the sleep-wake cycle is not synchronized with the circadian rhythm, for example after time-zone transitions or during irregular schedules of work and rest.

African American adolescents: adaptational processes and socioeconomic diversity in behavioural outcomes.

The study was concerned with determining the relationship of economic diversity to stress reactivity. Parental ratings of competence and behavioural problems were obtained for very young adolescents following a long-term and highly acute stress: the Atlanta youth murders. Intellectual and identity-formation data obtained both before and after the crisis were analyzed with the parental ratings of youth competence and behavioural problems. Consistent socioeconomic status (SES) links between the manifestation of behavioural problems and the display of competencies were obtained; lower income (LI) youths manifested more problems and fewer competencies. The findings were more pronounced for LI males. Personal identity was a significant predictor of school competence. School self-esteem was a predictor of academic performance. The manifestation of behavioural problems was significantly predicted by cognitive performance variables. The findings are discussed in terms of variations in adaptational strategies mediated by socioeconomic status and psychological factors.

Irregularity of rest and activity: studies on circadian rhythmicity in man.

1. Rectal temperature, electrolyte excretion and performance were studied in young adults who followed an irregular pattern of work and rest for 9 days in an isolation unit. 2. In the analysis, effects evoked by the pattern of work and rest were separated from the oscillatory component, and rhythms for individual days were examined by the cosinor method. 3. During the schedule, rhythms no longer showed a period of exactly 24 h, and this effect was confirmed by studies using a repeated cycle of irregular work and rest and by studies using constant routines. 4. Temperature and urinary constituents differed in the strength and phase of their rhythms when corrected for evoked effects, as well as in the strength of the evoked effects themselves. 5. There was evidence of deterioration in performance during work periods which exceeded 9 h, but there was no evidence of progressive deterioration in performance over the 9 day schedule.

Nocturnal sleep and daytime alertness of aircrew after transmeridian flights.

The nocturnal sleep and daytime alertness of aircrew were studied by electroencephalography and the multiple sleep latency test. After a transmeridian flight from London to San Francisco, sleep onset was faster and, although there was increased wakefulness during the second half of the night, sleep duration and efficiency over the whole night were not changed. The progressive decrease in sleep latencies observed normally in the multiple sleep latency test during the morning continued throughout the day after arrival. Of the 13 subjects, 12 took a nap of around 1-h duration in the afternoon preceding the return flight. These naps would have been encouraged by the drowsiness at this time and facilitated by the departure of the aircraft being scheduled during the early evening. An early evening departure had the further advantage that the circadian increase in vigilance expected during the early part of the day would occur during the latter part of the return flight.

Sleep after transmeridian flights.

Nocturnal sleep and daytime sleep latencies, recorded electroencephalographically after westward and eastward flights across the North Atlantic involving time zone shifts of 5 h, were influenced by the time of the flight and by subsequent displacement of the rest period. After the westward flight there was sleep disturbance during the latter part of the first night. However, there was persistent disturbance of sleep after the eastward flight. A rapidly eliminated hypnotic may be useful for the first night or two after a westward flight and for a few nights after an overnight eastward flight.

Thresholds for the detection of the direction of whole-body, linear movement in the horizontal plane.

Thresholds for the detection (at p = 0.67 correct) of the direction of discrete linear movements in the horizontal plane, having a cosine bell velocity trajectory and duration of 3 s, were determined in 24 subjects. Thresholds in the Z body axis (mean 0.154 m X s-2) were significantly higher than thresholds for movement in the X (mean 0.063 m X s-2) and Y (mean 0.057 m X s-2) body axes. In 8 subjects, X axis acceleration threshold was found to increase as a monotonic function of stimulus duration over the range 0.98 to 6.96 s and exhibited similar frequency-dependent characteristics to thresholds for the detection of continuous oscillatory stimuli. This finding implies that the sensory system mediating the transduction and perception of liminal, whole-body linear movement is sensitive to a combination of the acceleration and rate change of acceleration (jerk) of the motion stimulus, and has similar dynamics to the "irregular" sensory receptors of the otolith organs.

Performance overnight in shiftworkers operating a day-night schedule.

Performance was measured during the day (0800-1700 hours) and during the night (1700-0800 hours) of a day-night schedule, and the effect of caffeine (300 mg) was studied during the overnight periods of work. The sleep electroencephalogram was recorded together with oral temperature and urinary electrolyte excretion. Impairment of performance within 9 h after the beginning of the daytime work period was minimal, and was limited to a test of continuous performance, but impairment of performance within 9 h after the beginning of the overnight work period was more pronounced and included lowered vigilance. Impaired performance overnight was related to time on task and circadian rhythmicity, and was alleviated to some extent by the use of caffeine.