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Sleep and dreams
Our Biological Clocks

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Diane Boivin

If you work at night, you run a greater risk of suffering from diabetes, obesity, and cardiovascular disease. For example, one Swedish study has shown that the more people eat their meals at night, the higher their blood cholesterol levels, even if they eat exactly the same diet as people who are active in the daytime!

One factor that might be involved in this phenomenon is cortisol, a hormone whose secretion peaks just before you wake up. Cortisol increases your body’s temperature and prepares it to digest your breakfast. Thus, at 3:00 or 4:00 AM, when people who work night shifts typically eat their main meals, their cortisol concentrations may not yet be at their optimal levels. Their bodies simply may not be prepared to receive and process the food and hence may not metabolize it properly.


The human biological clock has evolved over millions of years so that we are adapted to the alternation of day and night. Hence this biological clock does not naturally facilitate activity during the nighttime, and the major changes that night workers must make in their periods of sleep and wakefulness cause a misalignment between their activity/rest cycle and their endogenous circadian oscillator.

This oscillator governs most of the body’s natural rhythms, including those of sleep cycles, physiological functions, hormone secretion, and alertness. When people have to operate against their normal circadian cycles for extended periods, all of these functions can be disturbed.

Why is it so hard for night workers to simply invert their internal biological clock and adjust to nighttime activity? The reason seems to lie in the cues that they receive from their external environment, and in particular, from daylight. The artificial light in a typical office or factory produces illuminance values of about 300 to 400 lux, but that is very little compared with daylight, which produces values about 50 times higher at dawn and up to about 500 times higher at noon. Thus if night workers are exposed to even just a small amount of daylight (for instance, while travelling to or from work), that seems to be enough to keep their internal clocks synchronized to the natural day/night cycle of the outside world. Apparently, this clock always synchronizes itself with whatever light source is sending it the strongest signals.

Knowing this, researchers have conducted numerous experiments to try to “trick” the body’s biological clock, usually by increasing the intensity of the artificial light to which night workers are exposed on the job while minimizing their exposure to daylight everywhere else. And the trick works fairly well: just as long-distance air travellers adjust to their destination time zones essentially by being exposed to the daylight there, these night workers adjust their biological clocks to the artificial light in their brightly lit workplaces–so successfully that their alertness peaks at night while they are at work, rather than in the daytime, when they are asleep in their darkened homes.

A typical example of one of these studies dealt with nurses working regular night shifts at hospitals in Quebec. These workers were divided into two groups. The test group were exposed intermittently to bright, full-spectrum white light for the first six hours of their shifts. They were also asked to wear dark glasses while travelling home and, once they got there, to remain in darkness for eight hours.

Meanwhile, the control group continued to work in their usual lower-intensity light. They too were asked to remain in darkness for eight hours after they got home.

After the members of the test group had worked their regular night shifts for about three weeks, they had adapted to their nighttime schedules completely, as measured by their circadian curves for physiological parameters such as body temperature, melatonin level, and salivary cortisol. The control group displayed only partial adjustments in all these parameters. The members of the test group also slept significantly longer during the daytime than their counterparts in the control group.

Such studies show that a carefully calibrated schedule of exposure to light and darkness can help workers adjust their biological clocks more completely to nighttime work while improving their daytime sleep. Employers should act on such findings to reduce the effects of night shifts on workers’ health.

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