Circadian Rhythms: The body has a timer
The lives of all human beings obviously are organized around the 24 hour solar day. The term usually applied to this rhythm is circadian, which means ‘about a day’. As it turns out, each person’s biological clock keeps their sleep and wake schedule synchronized, so called entrained, to the 24 hour light-dark cycle. If a person is placed in ‘temporal’ isolation, (ie in a cave or kept in dim light completely unaware of whether it is day or night), then the timing of a person’s sleep will dissociate from the setting and rising of the sun and “drift” to a different time. The amount of drift depends on how long the person’s “day length” or “tau” is. This is the amount of time required for one cycle of their circadian rhythm as measured by the production of melatonin. Melatonin is the “darkness” hormone that signals to various structures in the body that it is night. This cycle produces the circadian rhythm and dictates the duration of the biological “day” length. Humans have a biological “day” length of 24.2 hours on average with about 1/3 of the population less than 24 hours and 66% or more remaining greater than 24 hours. To stay ” in synch”, the circadian clock is influenced by multiple time givers or “zeitgebers” including light exposure and activity level. Multiple functions including alertness, vigilance, cognitive performance, temperature and digestive function and even sleep structure are influenced by the circadian rhythm. People normally retire as body temperature is falling, which it continues to do until the early AM, rising again in preparation for awakening in the morning.
The “body clock” is located in the suprachiasmatic nuclei (SCN); strategically located in the front portion of the base of the brain, not far behind the eyes, in an area termed the hypothalamus. In fact the ‘SCN’ is connected to the eyes by special nerve fibers, the retinohypothalamic tract. This provides these key brain structures with powerful light inputs. Light is the most important of the many time cues or ‘zeitgebers’ that keep the individual synchronized to the light-dark cycle. The SCN in turn controls the production of melatonin from the pineal gland. This hormone begins to appear as darkness nears, rising through the night to peak in the early morning hours and declining again to the usual minimal daylight levels by dawn. Its appearance is one of the best ways of “telling what time it is” in the brain, currently of great importance in sleep research and likely will also be so in the evaluation of circadian rhythm disorders.
The nature of the circadian clock is the basis for jet lag, in which the flight across times zones, especially going east, leaves the brain temporarily out of alignment with the new location. Human circadian mismatch is also the basis for impairment and sleepiness seen in shift workers and “night owls” that struggle with difficulty falling asleep and waking on time in the morning. This very primitive rhythm can adapt about 10 to 20 minutes per day with precisely timed light exposure and exercise. It is however, physiologically incapable of keeping pace with the demands of modern life such as light exposure or computer use late at night or travel across multiple time zones.
It is essential to emphasize that to be normally alert, a person must not only obtain a sufficient amount and quality of sleep, but must obtain that sleep at the appropriate time and be active when the biological clock is generating its wakefulness message. You also need to remember that a person’s estimate of the amount of sleep can be very inaccurate, especially when the quality is poor.
