A prospective study of fatal occupational accidents -- relationship to sleeping difficulties and occupational factors.

Very little is known about the association between sleep and (fatal) occupational accidents. This study investigated this relationship using register data of self-rated sleep difficulties, together with occupational and demographic characteristics. The variables were related to subsequent occupational fatal accidents. A national sample of 47,860 individuals was selected at regular intervals over a period of 20 years, and interviewed over the phone on issues related to work and health. The responses were linked to the cause of death register (suicides excluded) and the data set was subjected to a (multivariate) Cox regression survival analysis. One hundred and sixty six fatal occupational accidents occurred, and the significant predictors were: male vs. female: relative risk (RR)=2.30 with a 95% confidence interval (CI) of 1.56-3.38; difficulties in sleeping (past 2 weeks): RR=1.89 with CI=1.22-2.94; and non-day work: RR=1.63 with CI=1.09-2.45. No significant effect was seen for age, socio-economic group, hectic work, overtime (>50 h per week), or physically strenuous work. It was concluded that self-reported disturbed sleep is a predictor of accidental death at work, in addition to non-day work and male gender.

Sleep loss and performance: no "safe" duration of a monotonous task.

The capacity to maintain vigilance generally falls with time on task. However, it has been suggested that, for example, the effects of sleep loss need a rather long time on task to become evident. The present experiment examined when significant deterioration of performance occurred in a 34-min visual vigilance task (with 32 signals) given to twelve subjects every 3 h across 64 h without sleep. Results from the whole test, the first eight signals, and even the first signal varied significantly across the experiment and were significantly lower than baseline after 24 h awake. The rate of decline over time on task was similar across the experiment. Less than half the misses could be attributed to electrophysiologically defined sleepiness. It was concluded that there is no "safe" duration of a monotonous task if the situation is undemanding and boring, but that the effect may become immediately evident. This may have practical implications in terms of safety.

Subjective alertness and sleep quality in connection with permanent 12-hour day and night shifts.

OBJECTIVES: The aim of this study was to compare permanent 12-hour day and night shifts (shift change over times at 0500 and 1700) in a shift system with 3 work periods followed by 4 free days. METHODS: Sleep diaries were collected after main periods of sleep, and sleepiness ratings [Karolinska sleepiness scale (KSS)] were obtained 4 times during the last free day and also during the following 3 workshifts. Eighteen to twenty night workers and 8-10 day workers (depending on the instrument) participated. RESULTS: The day workers were significantly sleepier during their workdays. Times for going to bed and for rising differed between the groups. The amount of sleep per week did not differ between groups, but the pattern across days did in that the day workers had a short sleep (5 hours) before the first day and 6 hours of sleep after the other two. Night workers slept long (9 hours) before the first shift and had 6.5-hour sleep periods after the other shifts. During free time the day workers slept around 9 hours and the night workers around 8 hours. Sleep quality and ease of awakening showed no group differences in overall levels, but the day workers had difficulties awakening before their shifts. The night workers had little variation in sleep quality or difficulties awakening. CONCLUSIONS: The suggested explanation for the greater sleepiness and difficulties awakening among the day workers was the early start of the shift and the difficulties the workers had with phase advancing their sleep-wake rhythm.

Human sleep/wake regulation.

The present paper gives a short overview on what is known about human sleep/wake regulation and focuses on two main component, the homeostatic component and the circadian. The homeostatic components increases sleep need exponentially as a function of prior wakefulness whereas the circadian component is a consequence of the 24-hour physiological rhythm facilitating sleep during the night and counteracting sleep during the day. Normally, these components interact, in an additive way. This interaction has been described in mathematical models that can be used to predict sleep duration, depth of sleep and the level of alertness given the knowledge of circadian phase and prior time awake.

The effects of a short daytime nap after restricted night sleep.

Eight subjects participated on three occasions in a study investigating the effect of a 30 minute daytime nap opportunity on alertness/sleepiness. The baseline condition was a normal home sleep (7.5 hours, with bedtime at 2300 hours). Sleep during the other two conditions was between 2400 hours and 0400 hours. During one of the two 4-hour conditions, a short nap was allowed (between 1045 hours and 1115 hours). Self-ratings of sleepiness/alertness (Karolinska Sleepiness Scale) were recorded every hour. At 10, 12, and 15 hours, the subjects performed a 28-minute visual vigilance task. Electroencephalograms (EEG) and electrooculograms (EOG) were recorded continuously, including during a 10-minute standardized recording procedure at the beginning of each day. Mean total sleep time during the nap was 19.8 (standard error 2.4) minutes. Compared to baseline, EEG/EOG sleepiness and subjective sleepiness were significantly higher and vigilance performance at 10 hours lower, respectively, after the two short sleeps. The nap brought performance to baseline levels, and subjective sleepiness decreased significantly. It was concluded that the short nap had a clear positive effect on alertness.

Sleepiness and performance of professional drivers in a truck simulator--comparisons between day and night driving.

Previous research has shown that night driving performance may be seriously affected by sleepiness. The present study compared daytime and night-time performance of professional drivers on a simulated truck driving task. A secondary purpose was whether a nap or a rest pause would affect performance. Nine professional drivers participated in a counterbalanced design. The conditions were day driving (DAYDRIVE), night driving (NIGHTDRIVE), night driving with a 30 minute rest (NIGHTREST), and night driving with a 30 minute nap (NIGHTNAP). Each condition consisted of three consecutive 30-min periods. For the DAYDRIVE and NIGHTDRIVE all periods were spent driving while the second period was either a rest pause or a nap for the other two conditions. Mean speed, standard deviation of speed and, standard deviation for lane position were recorded. Self ratings of sleepiness were obtained before and after each 30-min period. Reaction time tests and 10 minute standardized EEG/EOG recordings were obtained before and after each condition. EEG/EOG were also recorded continuously during driving. The effects on driving were small but significant: night driving was slower, with a higher variability of speed, and had higher variability of lane position. Subjective and EEG/EOG sleepiness were clearly higher during the night conditions. Reaction time performance was not significantly affected by conditions. Neither the nap nor the rest pause had any effect.

Sleepiness and its relation to the length, content, and continuity of sleep.

This review of experimental studies focuses on the disturbances of sleep that may occur in connection with irregular work hours and their relation to ensuing alertness. Three aspects of sleep are covered: the duration of sleep, the stages of sleep, and the continuity of sleep. A sleep curtailment as small as 2 h has clear negative effects on alertness. Sleep disturbed as frequently as every minute clearly affects alertness, but such procedures also affect the stages of sleep and decrease total sleep time. However, fragmentation rates of one per 10 minutes also induce sleepiness without affecting sleep content or duration. There is no clear evidence for slow-wave sleep (SWS) being more important than other stages, but designs may not have been sensitive enough. In connection with fragmentation studies it is suggested, however, that the effects are stronger if more SWS is lost. In summary, the review suggests that the relatively mild disturbances of sleep in shift work may contribute to reduced alertness although there are probably other, more potent, factors.

Sleep restriction and SWS-suppression: effects on daytime alertness and night-time recovery.

This study evaluated tile effects of sleep curtailment and SWS-suppression, respectively, on daytime alertness and subsequent night sleep. Seven subjects participated in four conditions: an undisturbed 8-h sleep (8U; 23.00-07.00 hours), an undisturbed 4-h sleep (4U; 03.00-07.00 hours), a 4-h sleep (4D; 03.00-07.00 hours) that was acoustically disturbed when delta waves appeared, and a condition with no sleep (0). Subjective sleepiness, sleep latency, and simple reaction time (RT) were measured. In addition, sleep quality was rated. 4D contained 50% of the SWS (as well as spectral slow-wave energy; SWE) compared to 8U, whereas the curtailment to 4-h did not significantly decrease SWS. 4D had lower subjective quality than the other two sleeps. The main difference in daytime sleep latency was between the 8U and the 0 conditions. Rated alertness was highest after the 8U sleep. The two 4-h sleeps did not differ significantly with respect to rated sleepiness or sleep latency. However, the effects of the 4U sleep were closer to those of the 8U sleep and the effects of the 4D sleep were closer to those of the no sleep condition. RT performance was significantly better during the 8U condition. Recovery sleep after 4D sleep contained significantly more SWS than recovery after 4U and 8U. The effects on SWE during recovery were less clear. It was concluded that sleep duration might be more important for daytime alertness than SWS content and that loss of SWS during one night is recovered during the following night.

Relations between performance and subjective ratings of sleepiness during a night awake.

The present study validated the nine-point Karolinska Sleepiness Scale (KSS) and the new Accumulated Time with Sleepiness (ATS) scale against performance of laboratory tasks. The ATS scale was designed as a method for integrating subjective sleepiness over longer time periods. The subjects were asked if certain symptoms of sleepiness had occurred and, if so, for how long. Six subjects participated twice. Each time they were kept awake during the night (except for a short nap occurring during one of the nights in a counterbalanced order) and were tested at 2200, 0200, 0400 and 0600 hours. The tests included a 10-minute rest period, a 28-minute visual vigilance task and an 11-minute single reaction time task. KSS and visual analogue scale (VAS) ratings were given before each test, and ATS ratings were given after. Performance deteriorated clearly, and all three rating scales reflected increased sleepiness with time of night. Scores on the KSS and VAS showed high correlations with performance tasks (mean intraindividual correlations were between 0.49 and 0.71). Performance correlated even higher with the ATS ratings (r = 0.73-0.79). Intercorrelations between rating scales were also high (r = 0.65-0.86). It was concluded that there were strong relations between ratings of sleepiness and performance, that the ATS rating scale was at least as good as the other scales and that the ratings were affected by type of task.

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.

The dynamics of the first sleep cycle.

Eight subjects participated in an experiment in which sleep stages and electroencephalographic (EEG) power density during the first sleep cycles (and where such appeared, also second cycles) were studied in a design involving 8, 4, 2 or 0 hr of progressively postponed night-time sleep. Each of these four manipulations was followed by a day-time sleep beginning at 1100 hr. No significant changes in the duration of the first sleep cycle appeared. As the prior sleep loss increased both SWE (slow-wave energy; accumulated EEG delta power density) and SWA (slow-wave activity; EEG delta power per minute) increased during the 1100-hr sleeps. This was observed for the entire cycles, the nonrapid eye movement (NREM) periods, and the SWS periods, respectively. SWS latency decreased and SWS duration increased, respectively, markedly with prior waking. Also, for the progressively postponed sleeps (started at 2300 hr, 0300 hr, 0500 hr and 1100 hr) there were changes, but not as clear. After 28 hr of continuous waking there was a marked increase of SWA during SWS. Also, at this level there was a spill over of SWA to the second cycle. It is suggested that there might be a limit to the amount and intensity of SWS that can be accommodated in the first sleep cycle and that this limit is reached before the appearance of REM sleep.

Recovery within day-time sleep after slow wave sleep suppression.

Six subjects had their SWS activity suppressed by acoustic stimulation during a day-time (11.00 h) recovery sleep after a 4 h night sleep (03.00-07.00 h). Sleep was disturbed for a period corresponding to 90% of the duration of a preceding undisturbed baseline sleep (also at 11.00 h and preceded by a 4 h night sleep) and thereafter allowed to continue undisturbed until spontaneous awakening. The results showed that SWS and EEG power density were significantly reduced during suppression and that full recovery occurred before spontaneous awakening. The disturbed sleep was significantly longer than the baseline sleep. The increase in duration consisted mainly of SWS, stage 2 and REM. The results suggest that the suppression of SWS activity caused a need for an extension of sleep in order to allow recovery.

Subjective and objective sleepiness in the active individual.

Eight subjects were kept awake and active overnight in a sleep lab isolated from environmental time cues. Ambulatory EEG and EOG were continuously recorded and sleepiness ratings carried out every two hours as was a short EEG test session with eyes open for 5 min and closed for 2 min. The EEG was subjected to spectral analysis and the EOG was visually scored for slow rolling eye movements (SEM). Intrusions of SEM and of alpha and theta power density during waking, open-eyed activity strongly differentiated between high and low subjective sleepiness (the differentiation was poorer for closed eyes) and the mean intraindividual correlations between subjective and objective sleepiness were very high. Still, the covariation was curvilinear; physiological indices of sleepiness did not occur reliably until subjective perceptions fell between "sleepy" and "extremely sleepy-fighting sleep"; i.e. physiological changes due to sleepiness are not likely to occur until extreme sleepiness is encountered. The results support the notion that ambulatory EEG/EOG changes may be used to quantify sleepiness.

Catecholamine measurements in urine by high-performance liquid chromatography with amperometric detection--comparison with an autoanalyser fluorescence method.

In order to validate different methods of measuring urinary catecholamines (norepinephrine, epinephrine and dopamine) in humans, methods based on separation of catecholamines using reversed-phase or cation-exchange high-performance liquid chromatography with electrochemical detection were compared with an autoanalyser-based fluorescence method. Different methods for pre-chromatography sample purification were also studied. For measurements of urinary catecholamines, the reversed-phase-based chromatographic techniques studied were found to give less reliable results than cation-exchange chromatography, even if one of them (Clin Rep Urine Catecholamine Kit) gave almost as precise estimates. The autoanalyser technique yielded good results. It is concluded that cation-exchange chromatography with an appropriate sample work-up procedure (a combination of organic solvent extraction and alumina adsorption) is a reliable and accurate method for analyses of urinary catecholamines.

Military flight experience and sympatho-adrenal activity.

Urine excretion levels of adrenaline (A) and noradrenaline (NA) were determined, and achievement, commitment to the task, difficulty, risk, activation, and tension were rated after the preparation or planning and after the performance of 245 missions by 21 attack pilots. The catecholamine excretion levels increased and the ratio NA/A decreased as a function of condition (lesson, preparation, and mission). From a confirmatory factor analysis it was found that the catecholamine reactivity during preparation (values corrected for basal activity) was affected by the perceived challenge potential of the mission. The reference A activity covaried with the total A reactivity during the missions, i.e., the higher the basal excretion levels the higher the reactions to the missions. A positive relationship was found between former flight experience (hours) and mean activity of A. The rate of increase of A was potentiated by nicotine. Potential explanations of the increase of A are discussed.

Sleepiness in shiftwork. A review with emphasis on continuous monitoring of EEG and EOG.

Many forms of shift work disrupt the normal relation between rest/activity and the circadian regulation of bodily functions (1). Among the most obvious effects of this disruption is disturbed sleep and increased sleepiness (2, 3). Here we will discuss the effects on sleepiness. Much of the data in this area consist of self-ratings but our emphasis will be on some recent approaches that have made use of EEG methods to provide continuous monitoring of fluctuations of sleepiness in freely moving subjects.

Sleep duration and the power spectral density of the EEG.

In a sleep loss experiment with 8 subjects night sleep was restricted to either 8, 4, 2 or 0 h. The restrictions caused a strong dose-dependent increase in the duration of subsequent day sleep. Still, only a fraction of the sleep loss was recovered. Spectral analysis of the EEG demonstrated a strong dose-dependent increase also of power density. This parameter showed an almost perfect match between loss and recovery. Furthermore, daytime sleep duration was closely determined by the prior loss of EEG power density. Day sleep was maintained until the prior loss of power density had been recovered, at which point the sleep episode was spontaneously terminated. It was suggested that EEG power density reflects the homeostatic process in sleep regulation and that this process is a major determinant of sleep duration and content.

Urinary catecholamine responses to basic types of physical activity.

Urinary adrenaline, noradrenaline, heart rate, and subjective ratings were obtained from 9 healthy males during six different physical activities, ranging in intensity from lying down to running. Heart rate, subjective ratings and noradrenaline excretion reflected the work load in the different conditions. Adrenaline, on the other hand, failed to show this relationship. There was no significant increase in adrenaline excretion even at the highest work load (corresponding to a heart rate of 160 bpm). It was concluded that urinary adrenaline may safely be used as an indicator of mental factors even in situations with different levels of physical activity.

The effects of two alternative timings of a one-hour nap on early morning performance.

The effect on performance and sleepiness of two alternative timings of a one-hour nap (2100h and 0430h, respectively) were compared with a control condition (no nap). Twelve healthy male subjects divided into three groups participated in a partly balanced repeated measurements design. At all three occasions the subjects slept 4 h during the preceding night, worked during the day and were then kept awake (except for naps) in the laboratory from 1700h to 0800h the following morning. Performance was measured through a 10-min single choice visual reaction time task administered at 1900h and 0700h. Sleepiness was measured through self-ratings and sleep latency tests at 2100h and 0600h. The results showed clear positive effects of naps (especially the 0430h nap) on performance. The sleep latency measurements showed similar, but less clear tendencies, while ratings of sleepiness did not differentiate between conditions. It was concluded that a one-hour nap could counteract the late night performance decrement.