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Sleep and dreams

The Sleep/ Dream/ Wake Cycle

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Human beings are not designed to monitor nuclear generating stations at 4 in the morning–the task simply conflicts with their biological clocks. And the result is accidents such as those at Three Mile Island and Chernobyl.

Other famous accidents also have been attributed to impaired judgment and concentration due to working night shifts. Two examples are the Exxon Valdez oil spill in Alaska and the chemical gas leak in Bhopal, India.

Less famous are all the highway accidents caused by reduced alertness due to fatigue. It is estimated that nearly 40% of all highway accidents are caused by being sleepy or falling asleep at the wheel. It is also suspected that from 10 to 15% of all fatal accidents involving truckers are caused by fatigue. And these episodes of drowsiness or sleeping at the wheel appear to be eight times more common at night than in the daytime.

Alterations in people’s sleep schedules can cause them to experience significant mood swings even if they are otherwise in excellent psychological health, with no personal or family history of psychological disorders.


For countless millennia, our hunter-gatherer ancestors evolved in response to the naturally alternating rhythms of day and night and of the seasons. Being active after nightfall involved so many more risks than benefits that no one would have dared to attempt it. 

But then, somewhere between 800 000 and 400 000 years ago, humans learned how to control fire, which was very likely the discovery that led to more organized human societies. Fire not only was useful for cooking and for keeping warm, but also opened a breach into the nighttime realm of darkness.

The first system used exclusively for lighting was the oil lamp. The earliest oil lamps date back as far as 20 000 B.C. and consisted of a simple hollowed-out stone holding a wick in a pool of oil.

The Egyptians and the Cretans began making candles from bees’ wax as early as 3000 B.C. However, the first attempts to use candles for public lighting did not occur until the 16th century, when the Court of Paris, responding to public fear caused by the street attacks that were so common at night, published an edict encouraging citizens to place lighted candles in their windows from 9 in the evening on.

In 1829, the Rue de la Paix became the first street in Paris to be equipped with gaslights. But the real revolution in our relationship with the night came after Thomas Edison developed the incandescent light bulb in 1879. This invention extended the work day into hours that until then had always been devoted to sleep. A simple light bulb had made it possible for industry to operate 24 hours per day, and hence to require employees to work at night.

Later technological developments in the 20th century, such as intercontinental flight, also affected our natural sleep patterns, but none of these developments has had such a sweeping impact as night work.

Because companies have never stopped striving to increase their productivity, more and more employees now have to work at night or on irregular schedules. It is estimated that over 20% of the workers in industrialized countries now work irregular schedules, which inevitably disturb their sleep patterns. 

Very often, even workers who have been working at night for many years still get about two hours’ less sleep per night than the average person. Their bodies never succeed in adapting completely to this inverted lifestyle, and they therefore suffer from a chronic lack of sleep.

Many cyclical physiological phenomena occur in the human body that make it easier for us to be awake in the daytime and asleep at night. Thus night work forces our bodies to be active when they want to be asleep, and to try to fall asleep when several daylight-driven internal processes tend to keep us awake.

The poorer, shorter sleep experienced by night workers not only can cause health problems (such as chronic fatigue, ulcers, and indigestion) for these individuals, but also, because of the way it reduces alertness, can make these individuals and sometimes society as a whole more vulnerable to serious accidents (see sidebar).

Working night shifts on a regular schedule is generally distinguished from doing so on an irregular schedule, because each of these schedules has its own specific effects.

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