Timothy H. Monk

Towards a Predictive Test of Adjustment to Shift Work

A questionnaire was developed to distinguish between people who differed in the degree to which their circadian rhythms adjusted to night work. This was administered to 48 ‘ permanent ’ night nurses taking part in a large shift work study. Factor analyses indicated that there were three main factors. These were (I) rigidity/flexibility of sleeping habits, (ii) ability/inability to overcome drowsiness, and (iii) morningness/eveningness. Correlations were computed between the nursesscores on each of these factors and a range of measures of adjustment of circadian rhythm. A number of significant correlations were found with both psychological and physiological measures, thus indicating that the factors had at least concurrent validity. It is concluded that it may prove feasible to develop a questionnaire that would predict the degree to which peoples rhythms would adjust to shift work, and that flexibility of sleeping habits and the ability to overcome drowsiness should be components of such a questionnaire.

Shiftwork and Performance

The recent increase in the incidence of shiftwork has been accompanied by a change in the type of task typically performed by the shiftworker. The technological advances which have produced both these effects have meant that the shiftworker is more likely to be engaged in cognitive, mentally taxing tasks than the predominantly perceptual-motor ones typical of earlier times. Contemporary research on time of day effects in performance efficiency has indicated that these task changes may be crucially important in determining on-shift performance, and hence the choice of the individual and shift system that is most suitable. Evidence is reviewed on task demands, the effects of different shift systems, and the role of individual differences. A descriptive model is proposed in which on-shift performance is seen to be dependent on the type of task, type of shift system, and type of person, with the three factors interacting via the workers various circadian rhythms.

Short and long-term adjustment of circadian rhythms in 'permanent' night nurses

Two studies are described in which differences were found in the adjustment of the circadian rhythms of full and part-time night nurses to night work. A distinction was drawn between short-term adjustment that takes place over successive night shifts, and long-term adjustment that may take the form either of a permanent ‘flattening’ of the rhythm, or of a facilitation of short-term adjustment. The results from the first study indicated that even when the potential for greater short-term adjustment was controlled for, the full-time staff showed greater adjustment to night work. The second study examined these differences in long-term adjustment in greater detail. No evidence was found of a permanent ‘flattening’ of the full-timers’ circadian rhythms, although they showed clear evidence of adjustment even on the first of a period of successive night shifts. The full-timers also showed more evidence of adjustment from the first to the second night shift. It is suggested that these differences in long-term ad...

The Effect of Memory Load on the Circadian Variation in Performance Efficiency Under a Rapidly Rotating Shift System

Experimental shift work studios have typically found body temperature and performance efficiency to show very similar circadian rhythms. However, the performance tasks used have placed little, if any, reliance on short term memory. Studies of the variation in performance during the normal waking day have found performance on most tasks to improve over the day but that on short term memory tasks to decrease. The present paper reports an experimental study of the performance of two subjects on a rapidly rotating (-2-2) shift system. Three versions of a now performance test, each with a different memory load, wore administered four times per shift. With the low memory load version, performance showed a high positive correlation with body temperature and was poor during the night shift. However, with the high memory load version, performance was negatively correlated with temperature and was best during the night shift. It is concluded that future shift work studies must take into account the memory load of t...

Spring and Autumn daylight saving time changes: Studies of adjustment in sleep timings, mood, and efficiency

Various aspects of adjustment to Daylight Saving Time (DST) changes were investigated using two Spring and two Autumn studies. After both Spring and Autumn DST changes, although adjustment of times...

Adjusting to the changes to and from Daylight Saving Time

A GREAT deal of attention has recently been paid to the circadian rhythms that have been found to exist in a number of human physiological and behavioural processes. There are several ways in which such rhythms can be disrupted, and extensive studies have been made of the effects of prolonged isolation1, abnormal working hours2, and flight across several time zones3. The adoption of Daylight Saving Time (DST) is another possible cause of disruption, but seems to have been generally ignored in spite of the large number of people potentially affected. We have found that significant disruptions of behaviour occur during adaptation to the time change.

Memory based performance measures in studies of shiftwork.

The phase of the circadian rhythm in performance efficiency on a given task is known to be influenced by the memory load involved. Two experiments were performed to determine whether memory load also influences the rate at which rhythms adapt to the phase-shifts involved in (a) transmeridian flight and (b) a long period of nightwork. In the first study, high and low memory load versions of a performance test were given to a 25 y old female subject experiencing a 5h eastward change in time-zone. Differences were found both in the initial phase of the two versions of the test and in the rate at which this phase adapted to the new time. In the second study, two young male subjects, working 21 consecutive night shifts, were given high and low memory load versions of the performance test, and a calculations test, every 4 h around the clock. The results were similar to those of the first study: a cosinor analysis revealed that despite periods of arhythmicity there were large differences between the rate of adap...

Time of day and processing strategy in free recall

Consistent effects of time of day have been observed in digit span/sequence performance, and in the immediate memory for information presented in prose. However, studies using syntactically unstructured word lists have yielded inconsistent results. Three experiments were conducted that examined the free recall of 15 word lists. In all three, immediate recall from the pre-recency positions was found to be superior in the morning to the afternoon. This superiority disappeared after a 20-min delay (Experiment 1) and under articulatory suppression (Experiment III), but was unaffected by a manipulation that equated recall order (Experiment II). Immediate recall from the recency positions showed a W-shaped trend over the day (Experiment I) that might account for the inconsistencies previously reported. It is suggested that the changes in recall from the pre-recency positions may reflect a decrease in maintenance, and increase in elaboration, over the day. Such a change in processing strategy could itself reflect an increase in attentional selectivity associated with the changes in basal arousal level commonly held to occur over the day.

The Interaction Between the Edge Effect and Target Conspicuity in Visual Search

The term edge effect in visual search is used to describe the finding that targets appearing in the outer half of a display have longer search times than those appearing in the inner half. The interaction between the edge effect and target conspicuity was explored using two experiments. Under conditions of target uncertainty, it was shown that the magnitude of the edge effect was unaffected by target conspicuity. In addition, two manipulations of the edge effect indicated that as the edge effect decreased, the difference between the search times of inconspicuous and conspicuous targets increased. The results are explained in terms of a model based upon the size of the mesh of fixations used to cover the display. Two computer simulations confirmed that the size of this mesh and the magnitude of the edge effect are inextricably related.

Time of Day Effects in a Range of Clerical Tasks

A battery of eight clerical tasks, a typing test, and two questionnaires were given to 34 secretaries in 10 a.m. and 4 p.m. sessions. Afternoon performance was found to be superior in speed of information extraction, and in accuracy of proofreading and complex date sorting. Intertask differences appeared to outweigh interpersonal ones.