Exposure to bright light and darkness to treat physiologic maladaptation to night work

Article Abstract:

Working at night can cause disturbances in the sleep-wake cycle that lead to insomnia during the day and reduced alertness with fatigue while on the job at night. There are also medical consequences; people who rotate shifts over the long term have an elevated risk of developing heart disease, digestive problems, and in women, reproductive dysfunction. Even people who remain on a permanent night-shift for many years do not fully adjust. Researchers have discovered that the hypothalamus, a region of the brain, is responsible for regulating the body's circadian (24-hour) rhythms. For this reason, the hypothalamus has been called the pacemaker of the circadian rhythms. It was once thought that the hypothalamus ''set'' the daily rhythms according to the timing of social interaction, but the authors have recently demonstrated that it responds instead to darkness and light, and can actually be ''reset'' by up to 12 hours over a period of two to three days. In this study, eight young men were exposed to bright light at night and almost total darkness during the day in an attempt to improve their adjustment to night work. Circadian adaptation was measured by monitoring body temperature, which is normally at its lowest in the middle of the night; if the subjects were to adapt to night work it would shift to reach a 24-hour low during the day. This adaptation was in fact achieved. The low temperature occurred in the midafternoon after four days of the treatment, which consisted of exposure to bright lighting at night and darkness while sleeping during the day. Other physiologic signs confirmed the conclusions based on body temperature. The subjects also reported improved alertness and mental functioning at night. Therefore, manipulation of lighting conditions can correct the maladaptation of the human circadian rhythms to night work. (Consumer Summary produced by Reliance Medical Information, Inc.)

Hypothalamus

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Strategies for resetting the human circadian clock

Article Abstract:

The economics of modern civilization has created the need for night shift work, making humans the only species that attempts to override its natural sleep-wake cycle. It has been found that people who work at night are at increased risk for certain medical conditions, such as digestive disorders, cardiovascular disease, infertility and insomnia. The apparent cause of these problems is a chronic disturbance of the normal circadian (24-hour) rhythms. Surprisingly, even individuals who work nights and sleep days for many years, rather than rotating shifts, never fully adjust to this pattern. In additional to the physical consequences of night work, there are economic considerations, because at night employees are less productive and alert and have more accidents. For these reasons it is imperative that researchers discover ways that night workers can achieve better physiological adaptation to the reversal of the normal sleep-wake cycle. A breakthrough in this area was reported by Czeisler, et al. in the May 3, 1990 issue of The New England Journal of Medicine. The study demonstrated that the circadian rhythms could be successfully "reset", so that night would be the time of greatest alertness, by manipulating lighting conditions. The subjects were exposed to very bright light at work during the night, and their bedrooms were adapted to block out virtually all daylight so that they could sleep in darkness. After five days of this arrangement, complete physiological adjustment had been achieved. Many industries that depend on night work may find that improvements in lighting conditions are feasible and will yield health benefits for workers and financial benefits, in terms of enhanced productivity, for the employer. (Consumer Summary produced by Reliance Medical Information, Inc.)

editorial

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Suppression of melatonin secretion in some blind patients by exposure to bright light

Article Abstract:

The eyes of some totally blind persons may convey enough information about light and darkness to their brains to synchronize their internal circadian rhythms. A study of 11 blind patients who did not consciously perceive light and of six normal subjects found that three of the blind patients experienced decreases in plasma melatonin concentrations during exposure to bright light, as do sighted persons. The other eight blind patients did not experience changes in their plasma melatonin concentrations. Plasma concentrations of melatonin are found to be higher during nighttime than during the day, and are a way of synchronizing, or entraining, the circadian rhythm to the 24-hour day. Blind persons who do not perceive light may experience periods of severe insomnia because they do not receive the cues of light and dark to entrain their bodies to the 24-hour day.

Health aspects, Measurement, Blind, Blind persons, Melatonin, Pineal gland

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