Life Rhythms
All living things have natural internal rhythms that regulate cellular functions and physiological processes. These rhythmic cycles act as a type of clock that dictates when and how our bodies do almost everything — crave food, get thirsty, generate energy, stay alert, fall asleep, regulate mood, maintain body temperature and more.

In humans, the most prominent internal cycles are circadian rhythms. These rhythms are internally generated, but they are triggered and influenced by external stimuli, such as variations in temperature or travel across time zones. (Jet lag is what you experience while your circadian rhythms are realigning themselves.) The most powerful external influence on circadian rhythms, however, is exposure to light and dark.

As light fluctuates over the course of 24 hours, the brain tracks it through the retina’s ganglion cells in the back of the eye. When a photosensitive pigment in those cells, called melanopsin, detects light, those ganglion cells send information about the duration and brightness of that light to clusters of nerve cells, called the suprachiasmatic nucleus (SCN), in the brain’s hypothalamus.

The SCN is the master pacemaker of circadian rhythms, or, simply, the “master clock.” Among its other duties, it determines the daily rise and fall in body temperature (which is lowest around 5 a.m.) and blood pressure (which is often 20 percent higher in the late afternoon than it is in the morning). The master clock also tells the brain’s pineal gland when to produce melatonin, a sleep-promoting hormone with powerful antioxidant properties.

Melatonin production begins about two hours before you feel the urge to sleep at night and continues until shortly before you awake in the morning, says Michael Terman, PhD, director of the Center for Light Treatment and Biological Rhythms at Columbia University Medical Center. Whether you’re a night owl (you don’t feel drowsy until late at night) or a morning lark (you naturally wake up early) is a matter of genetics. “The way the clock adjusts to day and night is largely genetically determined,” says Terman, “and lark and owl chronotypes are at the extremes.”

Melatonin, explains Mark Rea, PhD, director of the Lighting Research Center at Rensselaer Polytechnic Institute in Troy, N.Y., “acts as messenger” between the master clock and several other peripheral circadian clocks in the body, such as those in the pancreas, skin and several internal organs, including the liver. “Each system in the body has its own rhythm that is coordinated by the master clock.”

Coordination between the master clock and peripheral clocks (crucial for letting us know what to do and when to do it) is dependent on the natural 24-hour light-dark cycle. If our exposure to light and dark becomes aperiodic, the master clock can lose control of the timing of the peripheral circadian clocks. Our internal rhythms become out of sync with each other, and it’s this internal rhythmic discord that causes trouble.

Next: Mixed Signals