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S Folkard

Publications and source records attributed to S Folkard.

At least 19 recordsLinked to original sources
PubMed1992

Sleep on a shortening day/night schedule.

In 2 experiments subjects were exposed for 3.5 weeks to a gradually (0.2 h/day initially) shortening day/night cycle, ending at 22.8 h and 22.0 h, respectively. Shortening of the cycle led to an initial but temporary increase of sleep latency. When the reduction ceased at 22.8 h and this length was maintained, sleep parameters were not further affected and the temperature rhythm in most subjects remained entrained to the 22.8 h period, although some instability occurred towards the end. In the 22.0 h experiment the continued reduction beyond 22.8 h led to disturbed sleep on day 15, at a day length of 22.4 h. Total sleep time, stage 2 and sleep efficiency were then markedly reduced. At this point sleep coincided with the peak of the body temperature rhythm and the amplitude of the latter was extremely small. This was also the point when the body temperature rhythm 'broke out' from the sleep/wake rhythm and showed a large 6 h phase jump (delay). Towards the end of the experiment, when sleep was initiated in the circadian temperature trough, REM propensity was increased. It was concluded that several sleep parameters were affected by the reduction of the day/night cycle although the specific effects depended on the amount of phase advance and on whether desynchronization occurred. Within the range of entrainment, however, most sleep parameters were remarkably unperturbed by the considerable changes of circadian parameters.

Adult↗
PubMed1992

Is there a 'best compromise' shift system?

In his review Wilkinson considers the question 'How fast should the night shift rotate? He concludes quite strongly that with the possible exception of social factors, most of the evidence favours the use of prolonged or permanent night shifts. In this reply I argue that in reaching this conclusion Wilkinson has (i) overestimated the problems associated with rapidly-rotating shift systems; and (ii) underestimated the problems that might be encountered in trying to implement effective permanent night shift systems. The evidence on the various problems reviewed by Wilkinson is reconsidered. Other aspects of shift systems that were ignored by Wilkinson are then also taken into account. It is concluded that the evidence does not allow a general choice to be made between the use of either permanent night shifts or rapidly rotating, delaying, shift system. Rather the choice between them would appear to depend on the relative importance attached to safety and social problems in any given workplace.

Circadian Rhythm↗
PubMed1991

"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.

Adaptation, Physiological↗
PubMed1991

Echinococcus multilocularis antigens modify accessory cell function of macrophages.

Peritoneal macrophages and splenic lymphocytes were collected from BALB/c mice, normal or previously infected with Echinococcus multilocularis. In an accessory cell function assay, peritoneal macrophages, in increasing numbers, were added to cultures of splenic lymphocytes. Cultures were stimulated by concanavalin A (Con A) or E. multilocularis culture supernatant (EMSN). Post-infection macrophages, unlike normal macrophages, suppressed Con A- and EMSN-driven lymphocyte transformation. Modification of accessory cells could also be repeatedly induced in vivo by EMSN or a single FPLC fraction of EMSN. Lymphocytes were made more sensitive to accessory cell signals following incubation with EMSN.

Animals↗
PubMed1990

Melatonin stabilises sleep onset time in a blind man without entrainment of cortisol or temperature rhythms.

The pineal hormone melatonin (N-acetyl-5-methoxytryptamine) is normally secreted at night: in animals it serves to transmit information about light-dark cycles to body physiology Suitable timed administration will alleviate 'jet-lag' severity ratings in humans, the major effect being to improve sleep. It has been suggested that this may be mediated by melatonin re-entraining the endogenous circadian oscillator. We have examined this possibility by feeding melatonin to a blind individual (HK) with a free-running temperature rhythm and a pronounced 35-day cycle in his ability to fall asleep at 'normal' times. Our results show a clear stabilizing effect of melatonin on sleep onset time with elimination of day time sleep, but no entrainment of rectal temperature or urinary cortisol rhythms. Thus melatonin may act on the timing mechanism of sleep onset, rather than as a entrainer of all circadian rhythms. It may well help shiftworkers to sleep at inappropriate phases of their circadian oscillators.

Blindness↗
PubMed1990

Circadian performance rhythms: some practical and theoretical implications.

Safety and productivity are low at night and this would appear to be because we are a diurnal species. This is reflected not only in our habitual sleep time, but also in our endogenous body clocks that, together with exogenous influences, such as the patterning of meals and activity, result in predictable circadian (24 h) rhythms in our physiological processes. Our performance capabilities also vary over the course of our waking period, with task demands affecting both the precise trend over the day, and the rate at which it adjusts to the changes in sleep timing occasioned by shift work. Studies designed to examine the reasons for this have shown that memory loaded performance may have a quite separate endogenous component to that responsible for more simple performance, suggesting that these two types of performance cannot be causally related. Furthermore, it would appear that the exogenous component of circadian rhythms may also differ across measures, and our attempts to model these endogenous and exogenous components have led us to re-examine the evidence on adjustment to night work. Our findings suggest that shiftworkers merely 'stay up late' on the night shift, rather than adjust to it, and that this is responsible for the reduced safety at night. It would seem that in situations where safety is paramount, the only solution to these problems is the creation of a nocturnal sub-society that not only always works at night but also remains on a nocturnal routine on rest days.

Circadian Rhythm↗
PubMed1989

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.

Adaptation, Psychological↗
PubMed1989

The pragmatic approach to masking.

This paper advocates the use of a pragmatic approach to the problem of masking in real-life situations involving an abrupt change in the timing of sleep, i.e. shiftwork and "jet-lag" situations. Although "pure" chronobiological research has pointed to the importance of taking masking effects into account, the techniques that it has provided for doing so are extremely difficult to apply in real-life situations. The approach advocated here is based on Wever's pioneering work, and involves estimating the normative endogenous and exogenous components of the circadian rhythm in body temperature. These estimates are then used to: (a) simulate the results of shiftwork studies; and (b) to "remove" the exogenous component in "jet-lag" studies to allow analysis of the estimated endogenous component. The simulated curves obtained cross-correlated extremely highly with published night-shift temperature curves, while the "removal" of the exogenous component resulted in very similar findings to those obtained in temporal isolation studies. It is concluded that this pragmatic approach to masking may prove extremely useful in interpreting the results of field studies of shiftwork and "jet-lag".

Body Temperature↗
PubMed1988

Sleep and circadian rhythms of temperature and urinary excretion on a 22.8 hr "day".

Two groups of subjects (total N = 6) were studied in an isolation chamber for a period of 3 weeks whilst living on a 22.8 hr "day". Regular samples of urine were taken when the subjects were awake, deep body temperature was recorded continuously and polygraphic EEG recordings were made of alternate sleeps. The excretion in the urine of potassium, sodium, phosphate, calcium and a metabolite of melatonin were estimated. Measurements of the quantity and quality of sleep were made together with assessments of the temperature profiles associated with sleep. In addition, cosinor analysis of circadian rhythmicity in urinary variables and temperature was performed. The 22.8 hr "days" affected variables and subjects differently. These differences were interpreted as indicating that the endogenous component of half the subjects adjusted to the 22.8 hr "days" but that, for the other three, adjustment did not occur. When the behaviour of different variables was considered then some (including urinary potassium and melatonin, sleep length and REM sleep) appeared to possess a larger endogenous component than others (for example, urinary sodium, phosphate and calcium), with rectal temperature behaving in an intermediate manner. In addition, a comparison between different rhythms in any subject enabled inferences to be drawn regarding any links (or lack of them) that might exist between the rhythms. In this respect also, there was a considerable range in the results and no links between any of the rhythms appeared to exist in the group of subjects as a whole. Two further groups (total N = 8) were treated similarly except that the chamber clock ran at the correct rate. In these subjects, circadian rhythms of urinary excretion and deep body temperature (sleep stages and urinary melatonin were not measured) gave no evidence for deterioration. We conclude, therefore, that the results on the 22.8 hr "day" were directly due to the abnormal "day" length rather than to a prolonged stay in the isolation chamber.

Adolescent↗
PubMed1987

Bright light phase shifts the human melatonin rhythm during the Antarctic winter.

In most species daily and seasonal changes in the light-dark cycle are the most important synchronisers (zeitgebers) of daily and seasonal rhythms. In humans only bright light (2500 lux) appears to be an effective circadian zeitgeber. Seasonal effects of light on human physiology have not been investigated. We have exploited the low intensity illumination of the Antarctic winter to investigate the effects of bright- or dim-light treatment for an hour in the morning and in the evening (a 'skeleton' 12.5-h day) for 6 weeks on the plasma melatonin rhythm, together with mood and a number of behavioural variables. In parallel seasonal changes in melatonin were observed. Melatonin is known to convey daylength information in photoperiodic seasonal breeders through characteristics of its night-time secretion profile. Bright-, but not dim-, light treatment in winter induced a marked phase advance of the melatonin rhythm, similar to that found in the summer, without marked effect on the other variables. Thus at least one human seasonal change appears to be light-dependent.

Antarctic Regions↗
PubMed1986

Our diurnal nature.

Explore the source record for details and available documents.

Biological Clocks↗
PubMed1986

Persistent 24-h variations of urinary 6-hydroxy melatonin sulphate and cortisol in Antarctica.

Bright light (2000-3000 lux) of sufficient intensity to suppress human melatonin secretion, acts as a strong zeitgeber in the entrainment of circadian rhythms in man. In polar conditions, light of this intensity is not experienced for several weeks during the winter. The entrainment of human circadian rhythms, in particular that of melatonin, is clearly of interest in these circumstances. Urinary 6-hydroxy melatonin sulphate (aMT6s) is a good index of melatonin secretion in man. In a limited study of seven male volunteers living on an Antarctic base the overall 24-h rhythm of aMT6s excretion was maintained at four different times of year (spring, summer, autumn and winter) and no significant seasonal effects were noted. Cortisol excretion, appeared to be markedly affected by the season although other factors such as social and environmental stress cannot be discounted. These observations suggest that in the absence of a strong light-dark cycle melatonin production may be entrained by other factors.

Antarctic Regions↗