Simon Folkard

Circadian characteristics influencing interindividual differences in tolerance and adjustment to shiftwork

The study was carried out to evaluate whether shiftworkers showing different long-term tolerance to shiftwork differ in their circadian adjustments and/or in some behavioural characteristics. Three groups of eight workers, engaged on three shifts in a graphic plant and matched for age and work experience, were selected according to the presence or not of complaints related to shiftwork: (1) no complaints; (2) nervous complaints (anxiety/depression, severe sleep disturbances); (3) digestive disorders (gastroduodenitis, peptic ulcer). They answered questionnaires on family conditions, health status, rigidity of sleeping habits, ability to overcome drowsiness, morningness, manifest anxiety. They also recorded several physiological parameters (oral temperature, grip strength, peak expiratory flow rate, pulse rate, sleep hours) during day and night-shifts. The data obtained indicate that the characteristics of flexibility of sleeping habits, ability to overcome drowsiness, and lower manifest anxiety, are associated with better tolerance to shiftwork. These characteristics do not seem to influence the adjustment of the circadian rhythm of oral temperature passing from day to night-shifts and vice versa. Conversely, morningness appeared to be unrelated to long-term tolerance, but did influence circadian adjustments and sleep behaviour. Among the groups, the subjects with digestive disorders showed a greater phase shift and a reduction of the amplitude on night-work, suggesting a possible relationship also between the short-term circadian adjustment and the long-term tolerance to shiftwork, as pointed out by other authors.

Recovery from work shifts: how long does it take?

Although regulations on work hours usually include a minimum weekly rest period, there is little empirical evidence concerning recovery from work. Shift-working nurses (N = 61) used a handheld computer for 28 days to complete self-ratings, cognitive-performance tasks, and a sleep diary. Many measures were worse on rest days that followed a night shift rather than a day shift and tended to be worse on first rest days compared with subsequent rest days. Alertness was lowest on the 1st rest day following a night shift. Social satisfaction was better on workdays that were preceded by 2 rather than 1 rest day. Reaction time decreased over consecutive night shifts and tended to increase over rest days following night shifts. The results are interpreted as being consistent with the combined adaptive costs of fatigue and adjustment to and from a nocturnal routine. The practical implications for scheduling rest days are considered.

Is there an optimum number of night shifts? Relationship between sleep, health and well-being

By concentrating on the impact of a specific shift-system feature on the well-being of those concerned, rather than on the impact of the shift system as a whole, one might be able to offer more meaningful suggestions as to what constitutes a better form of shift system. The present study focused on the impact of the number of consecutive night shifts worked on the health and well-being of two groups of nurses (permanent night and rotating shift). All nurses completed a copy of the Standard Shiftwork Index, which is a set of questionnaires designed for comparing the effects of different types of shift system on large groups of workers. It includes measurements of psychological ill-health, physical ill-health, chronic fatigue, social and domestic disruption, attitudes towards shiftwork, sleep quality and sleep habits. Results showed clearly the impact of the number of consecutive nights worked on health and well-being, not directly, but indirectly through the impact on sleep duration and sleep quality. Sleep duration was shown to increase with more consecutive nights worked. This in turn was found to predict sleep quality, which in turn was found to be the stronger direct predictor of psychological and physical ill-health i.e. better health was associated with longer and better quality sleeps. Explanations in terms of circadian adaptation are discussed.

Demasking the Temperature Rhythm after Simulated Time Zone Transitions

Simulated time zone transitions were performed in an isolation unit upon groups of one to four human subjects. In the first series of experiments, the adjustment of the circadian rhythm of body temperature, measured in the presence of sleep and other masking factors, was assessed by cosinor analysis and by cross-correlation methods. These methods modeled the circadian timing system either as a single component or as the sum of two components, those due to exogenous and endogenous influences. The one-component models described a more rapid adjustment of the temperature rhythm to the time zone transition than did the two-component models; we attribute this difference to the masking effects of the exogenous component. In a second series of experiments, we showed that the shift of the endogenous component, as assessed by the two-component models, was not significantly different from that measured during constant routines. The results also showed that, if the zeitgebers were phased in advance of the endogenous component, then advances of the endogenous component were produced only if this mismatch was less than about 10 hr. Mismatches greater than this, and cases where the zeitgebers were delayed with respect to the endogenous component, both produced delays of the endogenous component. We conclude that the two-component cross-correlation methods can be used to estimate shifts of the endogenous component of a circadian rhythm in the presence of masking factors. They are therefore an alternative to constant routines when these latter are impracticable to carry out.

Towards the Prediction of Alertness on Abnormal Sleep/Wake Schedules

This paper describes our recent progress in developing a mathematical model that may eventually allow the prediction of alertness on any given sleep/wake schedule. In its present form it comprises an endogenous sinusoidal function, and two exogenous logarithmic functions that are “set” on awakening. These functions are shown to simulate published time-series of alertness ratings with considerable accuracy.

Estimating the endogenous component of the circadian rhythm of rectal temperature in humans undergoing normal sleep/activity schedules

Abstract Continuous recordings have been made of rectal temperature for up to 23 consecutive days, using 13 subjects living a normal routine of sleep and activity. A series of purification methods have been developed that enable the direct effects of sleep and activity upon body temperature to be taken into account; this allows a daily estimate of the “unmasked”; circadian rhythm to be made. Results indicate that the purified, unmasked rhythm showed more daily phase variation than the raw (masked) data. These findings are discussed with regard to the stabilizing effect upon the measured circadian rhythm that masking factors have, and in terms of estimating the phase variability of the endogenous component of the circadian rhythm.

The Preferences Scale: Multinational Assessment of a New Measure of Morningness

Several self-report scales have been developed to measure morningness, or the preference for early morning or late evening activity (i.e., larks vs. owls). This individual difference in human circadian (time-of-day) rhythms has applications in the selection and placement of shiftworkers for night work in industry. Researchers have recently questioned the suitability of existing scales for people with alternate sleep-wake schedules and lifestyles. These factors may also render existing scales inappropriate for use in some cultures. To address this deficiency, we developed a new morningness scale (the preferences scale) that is not yoked to specific times of the 24-hour day and has a simplified response format. Data on the preferences scale were collected from university students (total N=1,212) in four countries (U.S., England, Holland, and Spain). Responses on external validity measures (self-rated alertness over the waking day and actual and preferred arise and bed times) and another morningness scale (for comparative purposes) were also collected. Although scale statistics (aggregate and by country) suggest that the psychometric properties of the preferences scale were adequate, scale mean differences existed across countries. These results may indicate cultural, as well as the established biological, differences in morningness. However, a more thorough psychometric assessment is needed to substantiate this possibility.

Sleep wakefulness cycle

Abstract Eleven subjects, in groups of 2 or 3, have lived in an isolation unit for a period of 30‐h “days”; (lOh sleep, 20h awake and sedentary). Rectal temperature and urinary excretion of potassium and sodium were measured throughout the experiment. Estimates of the phases of raw data indicated dissociation or desynchronization between the rhythms. When the data were “purified”;, however, the rhythms remained in phase with each other and showed a period in excess of 24h. The interpretive problems produced by such a protocol are discussed.

Aging and Shiftwork: A Process Model

With the aging of the workforce, and the increasing number of people who work shifts, understanding the experiences of older shiftworkers is becoming increasingly important. To improve such understanding, a model of adaptation to shiftwork was developed and tested in a group of older (over age 35) shiftworkers. The model hypothesized that individual difference variables (e.g., morningness, flexibility, languid&y) strongly influence the onset of social and domestic disturbances. Such disturbances, in turn, elicit certain coping behavior. Disengagement (i.e., passive) coping efforts, as well as unsuccessful engagement (i.e., active) coping behaviors, were expected to lead to fatigue and decreased job satisfaction. In turn, these strains were predicted to influence the reduction of emotional well-being. The model was tested using data collected from older nurses who worked rotating shifts in Great Britain and was generally supported. Implications of the present study are discussed, including potential interventions for older shiftworkers and future avenues for research.

The Desynchronisation of Rhythms and its Implications for Internal Clocks

Any overt circadian rhythm in humans is the sum of endogenous and exogenous components. There is now overwhelming evidence that the endogenous component is driven by an autonomous clock; thus overt rhythmicity in a human will continue in the absence of external rhythmic factors or when these have been modified. The exogenous component results from a combination of the rhythmic environment and rhythmic habits of the individual (see 1,2,3,4 for reviews).