Sleepiness : night work, time zones and activity

Abstract

Shift work and travel across time zones affect sleep and alertness negatively, and hence influence accident rate. Self-selected work hours, contextual, and environmental factors affect sleepiness, but it is unclear exactly how. It is important to fully understand the mechanisms behind sleepiness in order to be able to apply countermeasures and develop shift schedules and work situations more adapted to human physiology. In the thesis, five studies converge to bring focus on sleepiness in different situations.

Study I assessed the differences in sleepiness and amount of sleep in a two-pilot crew compared to a three-pilot crew, as well as comparing the outbound westward flight to the homebound eastward flight. Study II contrasted pilots in a morning-flight crew with those in an evening-flight crew with respect to sleepiness and sleep. Study 11 also investigated outbound westward flights with homebound eastward flights. Sleep was recorded with an actigraph and sleepiness ratings were obtained with a validated sleep diary in Studies 1 and 11. Study III aimed at investigating subjective sleepiness across and within the four possible watches of the six hours on/six hours off system by using an experimental approach in a bridge simulator. Sleepiness ratings were obtained with a validated sleep diary.

Study IV was a first attempt to describe the natural, short-term variation of subjective sleepiness across a normal working day, but also an attempt to obtain data with a systematic manipulation of activity for three hours during the same day. Sleepiness ratings were obtained with a validated sleep diary. Study V compared a rapidly rotating shift system, with frequently occurring quick returns, with a flexible shift system based on self-determined work hours on sleep quality, sleepiness, and health. The relation between work hour characteristics, indicating compressed or difficult rosters and subjective sleep and sleepiness within the flexible shift system group, was also examined. Data were collected with a questionnaire.

A reduction of crew size by one pilot in Study I was associated with moderately increased levels of sleepiness, which could mean an increase in accident risk and a decline in safety in long-haul transmeridian flights. There were only moderate differences between the morning and evening flights in Study II, although the elevated sleepiness levels on the evening flight suggest that flying during certain times of the day should be regarded with some caution. Different safety measures, such as allowing more time for napping, may decrease the safety risks involved with increased sleepiness during flights. The mean level of sleepiness during the night in Study III was higher than both during the day and evening. There was a clear time-on-watch effect on sleepiness for all watches except for the morning watch. Sleepiness was higher during time off duty compared to the start of the day, evening, and night watches.

Study IV found that there was a considerable variation of sleepiness depending on activity. In Study V, the ability to influence work hours was received positively by workers. The flexible shift group obtained more sleep in connection with the shifts, and they reported slower returns and long work shifts, but longer spells with night shifts, more consecutive workdays, and more often only one day off between work periods. This suggests that the shift workers in the flexible shift system selected compressed work hours, and that long periods with days off were given priority. Sleep/wake problems were associated with the attitude to work hours.

The present thesis shows that sleepiness and sleep disturbances are affected by work schedules. This is shown to be true for police officers, sea captains, and pilots. The thesis also shows that flights with two pilots involve more sleepiness, as do the homebound eastward flights and the evening flights. Furthermore, marine watch schedules are sensitive to sleepiness. However, sleepiness is also reduced during breaks, and physical activity reduces sleepiness. Finally, the ability to determine an individual schedule was not related to sleep and sleepiness, but the attitude to work hours was closely related to sleep and sleepiness.