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Circadian Rhythms: The Body's "Clock"

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Woman sleeping in bed
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All species have a timing mechanism, or 'clock,' that controls periods of activity and inactivity. These clocks are known as circadian rhythms and refer the cycle of physiological and biological processes that fluctuate on a roughly 24-hour timetable. You have probably noticed these tendencies yourself, feeling more energetic and alert during peak periods of the day and more lethargic and run-down at other times of the day.

While many people refer to circadian rhythms as a single process, there are actually a number of body clocks that oscillate throughout the day. For example, mental alertness tends to peak twice in a day at 9AM and 9PM, while physical strength tends to crest at 11AM and 7PM.

How Does Your Body "Keep Time?"

A tiny cluster of approximately 20,000 neurons in the hypothalamus controls your body’s many circadian rhythms. Known as the suprachiasmatic nucleus (SCN), this master control center is responsible for acting as your body’s internal pacemaker. While the exact mechanisms for how this process works are unclear, environmental cues are important. Sunlight is perhaps the most apparent, controlling our daily sleep-wake schedule.

So how does sunlight affect your circadian rhythms? As the sunlight decreases at the close of the day, the visual system sends signals to the suprachiasmatic nucleus. Next, the SCN sends signals to the pineal gland to increase the production of the hormone melatonin. This hormone increase helps reduced activity and makes you feel increasingly sleepy.

What Happens When There is No Sunlight?

There has been a considerable amount of research on what happens to circadian rhythms when natural sunlight patterns are interrupted. Clinical research has shown that individuals who are blind from birth frequently have difficulty with their sleep-wake cycle because of the complete lack of environmental light cues. Those who perform shift-work or travel frequently are also subject to having their natural circadian rhythms disrupted.

In some major studies of circadian rhythms, participants stayed in underground units for weeks or even months at a time. Deprived of all natural light cues, the circadian rhythms of these participants began to shift toward a 25-hour schedule rather than the standard 24-hour pattern. Additionally, many of the body’s previously synchronized circadian rhythms shifted as well. When exposed to environmental sunlight signals, many of the body's rhythms operate on a very similar schedule. When all natural light cues are removed, these body clocks begin to operate on completely different schedules.

A Few Key Points to Remember

  • Your circadian rhythms are tied to sunlight cues.

  • Disrupting these patterns can lead to poor or difficult sleep.

  • Without light signals, people tend to operate on a 25-hour schedule.

  • Circadian rhythms also impact body temperature, pain sensitivity, mental alertness, physical strength, and the senses.

Learn more about some of the theories about why people sleep as well as the stages of the sleep cycle.

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