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body clock

body clock We live in a world that changes its levels of illumination and temperature every day and night of our lives. To make the best of opportunities and to avoid the risks posed by this variability, living things have evolved internal timing mechanisms, called biological clocks, which co-ordinate their metabolism and behaviour with the daily rhythm of the world. The clock mechanism that organizes our cycles of activity and inactivity consists of a number of structures located in the depths of the brain. It sends to the body a variety of nerve and chemical messages that orchestrate our organ systems so that they operate harmoniously with one another as well as with external light and temperature.

The world is full of time-giving signals, particularly the level of light, which could directly rule the patterns of activity of the brain and the body. But remarkably, our body clock (and all other biological clocks) can keep very good time even in the absence of external cues.

The capacity of the human brain clock to maintain its own rhythmic beat was discovered by two German scientists, Jurgen Aschoff and F. Wever, who studied human volunteers, living for many weeks in an underground bunker, isolated from external time cues. Under these conditions, each subject manifested remarkably regular rhythms of body temperature and of sleep — wake alternations, with a time-cycle slightly longer than the normal 24 hours. In most cases, the intrinsic cycles of temperature and sleep were locked together in phase, but sometimes they were dissociated (internal desynchronization). Because the periodicity of the internal clock approximates to the 24-hour cycle of the natural world, it is called a circadian rhythm (circa-, about; dian-, day); and because it is independent of external signals, it is called a free-running rhythm.

Because the free-running clock is not precisely locked to the 24-hour cycle, it must be reset on a daily basis by signals from the outside world. This apparently inefficient system gives us the ability to deal with the natural variability of the diurnal rhythms of light and temperature. From day to day and from season to season, the times of sunset and sunrise change continuously. If our brain clock could not be reset, we could not deal with these variations, and, over a lifetime, the clock would surely drift out of phase with the rhythm of the world. This dynamic resetting of the body clock is called entrainment.

With its artificial lighting and heating systems, the modern world has obscured many of the biologically significant aspects of our body clock. But jet airplane travel reminds us, sometimes painfully, that we are still in the thrall of the primitive and potent timer in our brain. ‘Jet lag’ is the discomfort we experience when our brain clock only gradually resets in a new location. After crossing six to twelve of the Earth's time zones in a single flight, we may require 3–5 days to adapt our sleep and temperature rhythms fully to the local conditions of our destination.

Experiments on animals have shown that the body clock system is controlled by a tiny brain region called the suprachiasmatic nucleus, lying in the hypothalamus on each side of the brain, just above the crossing points (chiasma) of the optic nerves, which carry signals from the eyes to the brain. Nerve cells of the suprachiasmatic nucleus generate a continuous train of nerve impulses, with the regularity of a metronome. Even if these nerve cells are isolated from the rest of the brain and maintained for long periods in tissue culture, they still produce a stream of impulses, which gradually shifts in frequency, being faster during the period of the day when the animal would be active (at night for rats) and slower during the period when it would be asleep. This, then, is the heart of the free-running circadian rhythm. A small number of nerve fibres from the optic nerve enter the suprachiasmatic nucleus and provide direct information about the external light level, which somehow modifies the pattern of firing of the nerve cells to reset the clock. The cells of the suprachiasmatic nucleus send signals to other parts of the brain, especially the pineal gland, initiating hormonal and nerve signals that synchronize the body and the brain to the biological clock.

Allan Hobson

See also biological rhythms; hypothalamus; pineal gland; sleep.

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