11 March 2009SLEEP: NATURAL BODY CLOCK AND ULTRADIAN RHYTHM
Why do we have a biological clock? It has been suggested that, during the course of evolution, organisms have maximized their use of the environment so as to maximize their chances of survival. It has been shown that man’s efficiency varies during the 24 hour period. We perform best between 7 a.m. and 11 a.m. in the morning, and worst at 3 a.m. at night when most of us are sleeping. Hence, for man, sleep coincides with the time of lowest efficiency, which is at night. A phase map can be constructed for each bodily process within the 24 hour period. The phase map for body temperature shows that temperature is highest in the day and lowest at night. It has been suggested that the phase map of sleep coincides with the reduction of body temperature at night. Dr Charles Czeisler of the Harvard Medical School claims that he can shift a person’s circadian rhythm quickly by exposing them to strong light and thus resetting the body block.
What about a natural body clock? Do we have one that is not under the influence of the sun? In one experiment, conducted in 1972, a French cave explorer, Michel Siffre, lived underground in a Texas cave for seven months, away from all noises and civilization. He was not permitted to have any watches, clocks, radios, or televisions. In other words, his external cues for time were removed completely. Under these experimental conditions, without an external time cue, the body clock was free running. After a period of days the natural body clock would emerge. It was found that under free running conditions the human body clock was about 25 hours.
However, once he returned to the natural environment, the body clock was reset to 24 hours again. This resetting of the biological clock depends on external cues, the strongest being the change from dark to light.
Experiments have also shown that if we are placed in an artificially lit day of 19 hours and an artificially dark night of 9 hours, we can be trained to live in a 28 hour clock. In this case the biological clock gradually becomes a 28 hour clock instead of a 24 hour clock, and the phase map of temperature shows a maximum once every 28 hours instead of once every 24.
Understanding the biological clock is important for people who have sleep problems. If we wake up every morning at the same time and sleep at about the same time every night, we are helping to keep the biological clock accurate. If we sometimes read, watch television, or have wild parties late into the night, this irregular life style disturbs the accuracy of the biological clock, so that when we want to sleep we may not feel sleepy. By keeping irregular hours of sleeping and waking, we cause ourselves to experience a mini jet lag all the time. To help the biological clock work in our favour, we should wake up at the same time each day. We know that under free running experimental conditions, without any outside time cue, our natural biological clock is about 25 hours. Were we to let nature take its course, we would sleep about one hour late each day. After a few days, we would be sleeping a few hours later than our normal sleep time. To reset the biological clock to 24 hours, we must ensure that we wake at the same hour each day. Although we think we have little control over sleeping, we do have full control over waking up. Waking up at the same time each morning is now one of the most important disciplines recommended by most sleep experts for treating insomnia.
The study of biological clocks and rhythms has now evolved into a special science called chronobiology. Chronobiologists are not only interested in the biological rhythm of sleep but also in other biological functions such as the rhythm of hormonal secretion, urinary excretion, gastric function, body temperature, and the periodical fluctuation of human performance during the 24 hour cycle. Some chronobiologists are even interested in the biological rhythms in animals and plants.
Franz Halberg, a US chronobiologist, divided biological rhythms into three types. In 1959 he described the familiar circadian rhythm— its period is about a day. Then, in 1967, he described ultradian rhythms, which are biological rhythms with periods of less than a day. These include the 90 minute sleep cycle, also known as the REM/NREM cycle or Kleitman’s basic rest activity cycle (BRAC). Rhythms of the third type are called infradian rhythms. These rhythms have periods in excess of a day; the most familiar is the monthly menstrual period. These biological rhythms can be summarised as follows:
(1) Ultradian rhythms: less than a day
(2) Circadian rhythms: about a day
(3) Infradian rhythms: more than a day
The ultradian rhythm with its periodicity of about 90 minutes determines human performance and arousal state both day and night throughout the 24 hour period. This 90 minute cycle has also been found to operate in other human biological activities; for example, urinary volume and concentration, gastric contraction activity, pupil size under constant illumination, respiratory rate, and even heart rate. This rhythm exerts its influence both in the awake and in the sleeping state.
The periodicity of the ultradian rhythm varies from species to species. In man, it is about 90 minutes; in cats, 20 minutes; in rhesus monkeys, 60 minutes. The significance of this ultradian rhythm and the reason for its existence is still a mystery.
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