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S Folkard

Publications and source records attributed to S Folkard.

At least 55 records · Page 3Linked to original sources
PubMed1996

The shape of the endogenous circadian rhythm of rectal temperature in humans.

Fourteen healthy subjects have been studied in an isolation unit while living on a 30h "day" (20h awake, 10h asleep) for 14 (solar) days but while aware of real time. Waking activities were sedentary and included reading, watching television, and so forth. Throughout, regular recordings of rectal temperature were made, and in a subgroup of 6 subjects, activity was measured by a wrist accelerometer. Temperature data have been subjected to cosinor analysis after "purification," a method that enables the endogenous (clock-drive) and exogenous (activity-driven) components of the circadian rhythm to be assessed. Moreover, the protocol enables effects due to the circadian rhythm and time-since-waking to be separated. Results showed that the masking effects on body temperature exerted by the exogenous factors appeared to be less than average in the hours before and just after the peak of the endogenous temperature rhythm. This has the effect of producing a temperature plateau rather than a peak during the daytime. The implications of this for mental performance and sleep initiation are discussed.

Adult↗
PubMed1996

The difference between activity when in bed and out of bed. III. Nurses on night work.

Eight nurses have been studied during rest days and three successive night shifts. Measurements of wrist activity have been made and used to assess the extent to which the pattern of daily activity changes between control (rest) days and days involving night work. One analysis considered wrist activity during time spent in bed; this appears to decrease in parallel with the amount of time in bed that is lost during night work but, when this effect is corrected for, there is greater activity during time spent in bed in the daytime compared with control days (when time in bed is during the night). The dichotomy of activity (between lower values during time spent in bed and higher values when out of bed) also decreases if time in bed is during the daytime while on night shifts. These changes in the amount of wrist activity and the dichotomy between activity in and out of bed are related to the changed quality and quantity of sleep that has been measured by self-report questionnaires and the sleep EEG. It is concluded that results from wrist actimetry can provide valuable information regarding the process of adjustment to night work, and that its convenience (to subject and experimenter), coupled with the new analytical approaches described here, make it a viable method for field studies.

Adult↗
PubMed1996

A preliminary investigation into individual differences in the circadian variation of meal tolerance: effects on mood and hunger.

A forced desynchrony methodology was used to assess postprandial blood glucose in 9 female volunteers during a 3-h period following a mixed meal presented at four times of day (08:00, 14:00, 20:00, 02:00). The influence of time of day on the postmeal glucose responses was evaluated by calculating the area under the curve, largest increase, time taken to reach peak, and fasting level. Circadian variations in meal tolerance were found for the area under the curve and largest increase, responses were greater (indicating poorer meal tolerance) in the evening than the morning. Fasting blood glucose exhibited diurnal variation although in the opposite direction to meal tolerance; levels were higher in the morning than the evening. Time taken to reach peak levels was not modulated by circadian rhythmicity. Estimates of the timing of poorest meal tolerance and the magnitude of this intolerance were computed for each subject. Individual differences in the magnitude of meal intolerance were found to influence hunger and self-reported calmness. Subjects with good tolerance had rhythms in both calmness and hunger, which were not found in those with poor tolerance. Subjects with good tolerance also tended to rate themselves as feeling more calm. These mood and hunger effects may result from differences in insulin resistance, which is hypothesized to underlie the circadian variations in meal tolerance.

Adult↗
PubMed1996

The difference between activity when in bed and out of bed. I. Healthy subjects and selected patients.

The activity records of five groups of healthy or ill subjects have been measured for 4-26 days by an accelerometer placed on the nondominant wrist. These data, together with a record of times retiring to/rising from bed, have been used to produce a series of dichotomy indices for comparing the amounts of activity when in bed and out of bed. Reliable differences between individuals were found, with healthy subjects showing a greater degree of dichotomy than one subject with delayed sleep phase syndrome or three subjects with colorectal cancer. The method is convenient for extended data collection and offers the possibility of describing an individual's activity profile in a variety of circumstances.

Adolescent↗
PubMed1995

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.

Adult↗
PubMed1995

Validation of the S and C components of the three-process model of alertness regulation.

This paper summarizes work to validate and develop further the homeostatic and circadian component of a quantitative (computerized) three-process model for predicting alertness/sleepiness in daily living. The model uses sleep data as input and contains circadian and homeostatic components (amount of prior wake and amount of prior sleep), which are summed to yield predicted alertness on a scale between 1 and 16. The present validation was carried out using regression analysis, with sleepiness-related electroencephalographic parameters (alpha power density) from field and laboratory studies as criteria. The results showed that variations in alpha-power density in truck drivers, train drivers and laboratory subjects could be predicted with considerable accuracy (r2 > 0.70) from the model, as could subjective alertness. Levels < or = 7 on the 16-point scale were defined as critically low alertness. The paper also describes a simplified, graphic, paper version of the computation model, visualized as a two-dimensional "alertness nomogram". It is suggested that the studied components of the model may serve as tools for evaluating work/rest schedules in terms of sleep-related safety risks.

Adult↗
PubMed1995

The use of survey measures to assess circadian variations in alertness.

Current models of sleep regulation postulate both a homeostatic and circadian component and promise an understanding of disturbed and displaced sleep. Estimates of these components have traditionally required relatively cumbersome and costly measures, including sleep electroencephalograms and continuously recorded rectal temperature, but it has recently been demonstrated that they may successfully be based on frequent (e.g. 2-hourly) concurrent ratings of alertness. This paper examines whether similar results might be obtained using retrospective survey measures of alertness obtained from shiftworking nurses at a single sitting. These retrospective measures are shown to be sensitive to both time of day and shift, to have a high level of reliability even for relatively small sample sizes (e.g. 10) and to be valid predictors of more traditional concurrent measures of alertness. It is concluded that retrospective alertness ratings may prove to be an extremely cost-effective method for examining the trends in alertness in various groups, including those suffering from specific types of sleep disorder.

Adult↗
PubMed1994

Increased injuries on night shift.

Concern over safety has resulted in legislation by, for example, the Commission of the European Union, to limit the number of hours worked at night. There is, however, no direct evidence that injuries are more frequent at night. We analysed all 4645 injury incidents reported for a year on a rotating three-shift system in a large engineering company where the a-priori accident risk appeared to be constant. The relative risk of sustaining an injury was 1.23 (95% CI 1.14-1.31) higher on the night shift than on the morning shift, which showed the lowest incidence. The type of work (machine versus self-paced) affected the pattern and nature of injuries; for self-paced work the relative risk of more serious injury on the night shift compared with the morning shift was 1.82 (1.30-2.34).

Accidents, Occupational↗
PubMed1994

Effects on health of a change from a delaying to an advancing shift system.

OBJECTIVES: Shift work can lead to a range of problems for some people that seem to result from the disturbance of the circadian system, and can broadly be classified as: disturbances of sleep, impaired physical and psychological health, and disturbed social and domestic life. The main attempt to try to reduce these problems has focused on the design of the shift system, and the identification of the most problematic features of the shift system. One such feature is believed to be the direction of shift rotation. Systems that advance are thought to be more problematic than those that delay. The present study examines the change in the direction of shift rotation from a delaying to an advancing system on health and wellbeing. METHODS: Self reported measures of tolerance to shift work were taken two months before and six months after the change. These included sleep difficulties, gastrointestinal problems, psychological ill health, chronic fatigue, social and domestic disruption, job satisfaction, and satisfaction with the shift system. RESULTS: The change from a delaying to an advancing system resulted in an increase in sleep difficulties between successive afternoon shifts, but a decrease in social disruption. There was little evidence of impaired health on the advancing compared with the delaying system. CONCLUSIONS: The increase in sleep difficulties was thought to result from the undesired adaptation of the circadian system to night work, as a result of the afternoon shifts now following a series of night shifts, whereas previously they followed a series of morning shifts. The decrease in social disruption was thought to result from the specific sequence of the shifts and the discontinuous nature of the shift system, in particular, the long week-end off every third week. Lack of reported health related differences are explained in terms of the relatively unharmful nature of the shift system in question, and the relatively short time span over which the study was conducted.

Adult↗
PubMed1993

A classification of nursing and midwifery shift systems.

A classification of 122 shift systems worked by nurses and midwives in the larger general hospitals (400+ beds) in England and Wales was made. The systems were classified along two main dimensions: the degree of flexibility for shift rostering (either regular, irregular or flexible); and the speed of rotation between night and day work (either a permanent night shift or systems of fast or slow internal rotation). This resulted in nine possible categories of shift systems. The most common shift system was a flexible day shift with a permanent night shift. Other features of the systems are discussed, e.g. the start times and durations of shifts, and the relative influence of flexible rostering on these features. This classification is a prerequisite for a further research project aimed at identifying those features of shift systems which are likely to cause the least detrimental effects for the individual nurses concerned.

Adaptation, Psychological↗
PubMed1993

Can melatonin improve shift workers' tolerance of the night shift? Some preliminary findings.

The pineal hormone melatonin is potentially useful in the treatment of disorders, especially sleep disorders, associated with circadian rhythm disturbance. We have examined its effects on sleep, mood, and behaviour in a double-blind, placebo-controlled study of a small group of police officers working spans of seven successive night shifts. Compared to placebo, and to no treatment, melatonin (5 mg) taken at the desired bedtime improved problems related to sleep and increased alertness during working hours, especially during the early morning. In letter-target performance tests visual search speed and accuracy were either unchanged or slightly improved. Memory scanning speed and perception of mental load were adversely affected. This preliminary study suggests that melatonin has beneficial effects on sleep and alertness, but that its effects on performance need careful evaluation.

Adult↗
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↗
PubMed1992

Slow wave activity and prior sleep/wakefulness on an irregular schedule.

Eight subjects were exposed to a sleep schedule of 8, 4, 2, or 0 h of night sleep (awakening at 07 hours) followed by ad lib sleep from 1100 hours. Stepwise multiple regression analysis showed that SWA per cycle was closely related to the amount of prior waking, the amount of prior napping and the amount of prior sleep in the same sleep episode. The homeostatic 'factor S' of the three-process model of sleep regulation of Folkard and Åkerstedt predicted as much of the variance of SWA as did the three prior sleep/wake variables together. It was concluded that SWA is closely related to prior sleep/wake history and follows a course similar to that of recovery of alertness.

Journal Article↗
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↗