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Sleep and dreams
The Sleep/ Dream/ Wake Cycle
Our Biological Clocks

Help Link : Bosser à son rythme : Décollage horaire Link : Working nights may shift biological balance toward breast cancer Link : Shiftwork
Link : Bright Light Therapy Link : guidelines for decreasing the effects of shift work Link : L’horloge biologique des travailleurs de nuit Link : Travail posté et travail de nuit. XIVe Symposium international
Link : Rhythms of life: the biological clocks that control the daily lives of all living things

The effects that lack of sleep has on performance and perception are similar to those of alcohol. The cumulative fatigue resulting from a 12-hour work shift alters reaction time, logical reasoning, hand-eye co-ordination, and decision-making in the same way as a blood-alcohol level of about 0.05 grams per 100 millilitres.
Workers who are fatigued may still be able to perform simple, routine procedures, but if they have to deal with a new situation or a sudden emergency, they experience a lot of difficulties and often make mistakes.

In Canada, on the day after the switch to daylight savings time, which causes people to lose 60 minutes of sleep, the number of fatal accidents jumps to 7% higher than the daily average. This number then returns to its average level over the following days. Even more surprisingly, the opposite pattern is seen in the fall: the day after the return to standard time, when people get an extra 60 minutes’ sleep, the number of fatal accidents falls by 7%, and then rises back gradually to its average level.


With the advent of mass production and the pressure for a more flexible work force, many companies began making employees work rotating shifts and night shifts.

To serve this new population of night workers, some convenience stores and supermarkets began staying open 24 hours per day. These stores of course had to hire employees to work the extra hours, thus further increasing the cohort of night workers. This cohort also includes workers providing essential services that must be available at all times, such as hospital workers, police officers, and firefighters.

However, many sleep specialists now think that this proliferation of night work should be contained as much as possible, because we now know that working at night permanently affects the quality of sleep. Sleep, which is indispensable for restoring our physical and mental abilities, can be greatly disturbed by working irregular schedules or working at night.

The problems that people have in adjusting to night work resemble those associated with jet lag. But unlike air passengers, who can adapt to the time at their destination, night workers must continue to live in a society that operates in the daytime. People who come home from a night shift and try to go to bed in the morning are exposed to the light of the rising sun. Many other external cues further remind them that they are out of sync with their environment. These external stimuli induce abnormalities in the night workers’ circadian patterns of hormone secretion–abnormalities that persist even after they have been working night shifts for several years.

These discrepancies between the body’s internal rhythms and the cues that it receives from the outside world in fact result in two distinct problems.

First, people who work night shifts must constantly fight the body’s natural tendency to want to sleep at night. As they do their work, they are struggling against a physiological state of rest and hence cannot help performing less accurately and effectively.

The second problem is sleep deprivation: people who work at night do not sleep so well during the daytime, because the daylight places their entire physiology in a mode that favours activity. Some night workers may get only 5 or 6 hours of poor-quality sleep per night. As one might well imagine, this chronic sleep deprivation (see sidebar) not only increases the risks of workplace accidents, but also, in the long term, is accompanied by a significant increase in various types of physical and mental health problems, such as cardiovascular disorders and depression.

Night work can be organized in two different ways: steady night shifts on a regular schedule (for example, 11:00 PM to 7:00 AM, 5 days per week, every week) or rotating night shifts on a less regular schedule (for example, 11:00 PM to 7:00 AM one week, 7:00 AM to 3:00 PM the next, 3:00 PM to 11:00 PM the next, and so on).

When people work steady night shifts for long periods, their bodies can adapt to some extent, though their circadian rhythms will still be somewhat out of phase because of the influence of daylight. But when people work rotating night shifts, the disturbances in their circadian rhythms are more pronounced. Rotating among a day shift, an evening shift, and an overnight shift makes it very hard for the body’s internal cycles to become synchronized with the usual external cues, so the body must adjust itself perpetually.

A variety of strategies have been proposed for reducing the influence of these external cues so that people who work steady or rotating night shifts can get better sleep.

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