The prediction of the adaptation of circadian rhythms to rapid time zone changes

To what extent is circadian phase predictive of subjective jet lag in long-haul cabin crew pre- and post-trip?

Long-haul cabin crew regularly report misalignment between their circadian phase and the external world (i.e. jet lag). The extent to which changes in circadian phase relate to reported levels of jet lag remains unclear. The main aim of the present study was first to evaluate the relationship between objective (circadian phase) and subjective jet lag and second to explore the relative role of both subjective and objective psycho-behavioural factors in predicting the subjective experience of jet lag. Twenty-eight long-haul cabin crew completed questionnaires measuring diurnal preference, trip characteristics and subjective jet lag as a single and as a multidimensional measure. Sleep was monitored using actigraphy and urinary melatonin peak time was measured, at baseline (T1), e.g. before a long-haul trip and post-trip on the crew's first recovery day (T2). Subjective jet lag was also measured at both time points. At T1, later circadian phase related to increased unidimensional jet lag, however, a post-trip discrepancy was found between objective and subjective uni- and multidimensional jet lag measured at T2 and change from T1 to T2. After controlling for direction and size of circadian phase, increased uni- and multidimensional subjective jet lag was predicted by depressed mood states. The regression models including phase, diurnal preference, departure time on the outbound sector and arousal levels accounted for 28% of the variance in unidimensional jet lag and 53% of the variance in multidimensional jet lag. It was concluded that there is a discordance between objective and subjective jet lag post-trip. Further, subjective jet lag in long-haul cabin crew is better explained by mood impairment than circadian phase. The results are discussed with reference to the gap between subjective and objective jet lag and the role of psychology rather than just biology in the jet lag experience. The implications for improving health and safety in the workplace, through a better understanding of the role of human factors in the management of jet lag, are discussed.

Exercise as a synchroniser of human circadian rhythms: an update and discussion of the methodological problems

We review the literature on whether exercise is a synchroniser of human circadian rhythms, and highlight the specific methodological problems that are associated with this topic. In applied research, exercise has been investigated as a treatment for jet lag and shift-work problems. In these studies, there have been difficulties in controlling the characteristics of the exercise bout, the athletic status of research participants and exposure to other confounding synchronisers. Therefore, it is unclear at present whether exercise can help mitigate the problems associated with transmeridian travel and shift-work. In laboratory-based experiments, participants have exercised at various times of the day and the change in phase of various circadian rhythms has been measured. Although it is difficult to control for the considerable masking effects of exercise on these estimates of circadian timing, it is clear that nocturnal exercise can induce phase delays in the onset of melatonin. Reports of exercise-induced phase advances of the melatonin rhythm are rarer, as are any phase-shifting effects at all on the body temperature rhythm. In practical terms, the substantial levels of activity needed to obtain phase shifts may not be attainable by the majority of people. In mechanistic terms, the lack of agreement with the phase-shifting effects of bright light suggests that exercise is not exerting its effects via photic entrainment pathways. An alternative explanation may involve exercise-induced hyperthermia. Moreover, it is perplexing why exercise should have a different phase response curve to light, given that humans are diurnally active.

Circadian rhythms, athletic performance, and jet lag

Rapid air travel across several time zones exposes the traveller to a shift in his/her internal biological clock. The result is a transient desynchronisation of the circadian rhythm, called jet lag, lasting until the rhythm is rephased to the new environmental conditions. The most commonly experienced symptoms are sleep disorders, difficulties with concentrating, irritability, depression, fatigue, disorientation, loss of appetite, and gastrointestinal disturbance. Apart from the decrements in mental and physical performance directly consequent on such symptoms, competitive athletes are also exposed to the additional negative consequences of a shift from the optimal circadian window of performance. A brief summary of the possible negative effects of jet lag on athletic performance and potentially alleviating strategies is given.

Effect of a westward transmeridian flight on ambulatory blood pressure monitoring in normotensive subjects

OBJECTIVE

To evaluate the effects of a westward transmeridian flight over six time zones (from Milan to New York) on ambulatory blood pressure monitoring (ABPM) in normotensive individuals.

METHODS

Eighteen normotensive subjects (blood pressure < 140/90 mmHg), 11 men and seven women, of mean age 38.3 years, were studied. On the day of travel they underwent 26 h noninvasive ABPM (started at 1100 h); the take-off time was 1200 h and the landing time was 8 h later, at 1400 h New York time (2000 h Italian time). Subjects were requested not to sleep until 2300 h and to get up at 0700 h the following morning. The results were compared with those of a 26 h ABPM performed in Italy the week before during which they slept from 2300 h to 0700 h.

RESULTS

During the flight blood pressure and heart rate did not change compared with values during the corresponding time interval of the control day. After the landing, during the New York afternoon and evening (corresponding to the Italian sleeping time), blood pressure and heart rate remained unchanged, whereas during the night they decreased significantly, although their drop was less pronounced than that during the control day.

CONCLUSION

The results of this study indicate that the decrease in blood pressure during sleep is the result of sleep itself rather than of the actual time of day.

Sleepiness and shiftwork: individual differences

On-site studies of shiftwork show considerable differences in sleepiness between workers. Variation between individuals depends on different sleeping, coping and living habits of the workers, on possible sleep disorders, but also on more 'permanent' inter-individual factors that may influence sleepiness by changing either the circadian or homeostatic factors of sleep. The effects of circadian phase, age, sex, physical fitness, domestic and personality factors on shiftworkers' sleepiness and work-related accidents are reviewed. It is concluded that only a minor part of the variation in sleepiness between individuals can be explained. The present knowledge is not sufficient to carry out reliable 'selection' of individuals suitable for shiftwork. However, preventive advice and medical surveillance focusing on individual differences in sleepiness are recommended.

Light treatment for sleep disorders: consensus report. VII. Jet lag

Sleep disturbances are an all-too-familiar symptom of jet lag and a prime source of complaints for transmeridian travelers and flight crews alike. They are the result of a temporary loss of synchrony between an abruptly shifted sleep period, timed in accordance with the new local day-night cycle, and a gradually reentraining circadian system. Scheduled exposure to bright light can, in principle, alleviate the symptoms of jet lag by accelerating circadian reentrainment to new time zones. Laboratory simulations, in which sleep time is advanced by 6 to 8 h and the subjects exposed to bright light for 3 to 4 h during late subjective night on 2 to 4 successive days, have not all been successful. The few field studies conducted to date have had encouraging results, but their applicability to the population at large remains uncertain due to very limited sample sizes. Unresolved issues include optimal times for light exposure on the first as well as on subsequent treatment days, whether a given, fixed, light exposure time is likely to benefit a majority of travelers or whether light treatment should be scheduled instead according to some individual circadian phase marker, and if so, can such a phase marker be found that is both practical and reliable.