Explainer: Our bodies’ internal clocks

Biological clocks determine hunger, sleepiness and other daily rhythms

 This illustration shows how light that enters the eyes can send a signal to make sure the body’s master clock is reset daily. This ensures the internal pacemaker is always working in sync with the local time. 

 Nat.l Inst. of Gen’l Medical Sciences

People are precisely tuned to eat, sleep and wake at specific times. These predictable patterns are known as circadian rhythms. (In Latin, circa means “around,” and dian relates to “day.”)

Circadian rhythms are biological cycles that occur about once every 24 hours. Driven by an internal “clock,” these include waking and falling asleep. But outside factors can influence the cycles too. Among such factors: diet, stress and exposure to light.

Every living thing possesses an internal master clock. In people, it resides in the middle of the brain. It is called the suprachiasmatic nucleus (SU-prah-KY-as-mat-ik NU-klee-us). But don’t let the clock’s oversize name fool you. This timepiece is only about as big as a grain of rice. Still, it coordinates all of the body’s daily rhythms.

On its own, it doesn’t keep very accurate time. In some people and other life forms this clock may run a little fast. In others a bit slow. So this pacemaker must be regularly reset. In people and many living things, the sun does this.

The body’s clocklike rhythms regulate every biological process. And they do this in every life form, from bacteria to people. The circadian system “is probably one of the most ancient biological systems you can imagine,” says Paolo Sassone-Corsi of the University of California, Irvine. In fact, he says, it’s probably been at play as long as there has been life on Earth.

Circadian rhythms are linked to the chemical reactions that maintain life in every cell. Collectively, these reactions are known as the body’s metabolism. Those reactions vary a lot over the course of 24 hours, but their schedule on any given day differs little from another. This shows, he says, that “basically every single step of our everyday life is controlled by the clock.”

Light cues reach the body’s master clock through very specialized cells in the retina. This light-sensitive layer of tissue lines the back of our eyes. When light hits its cells, they dispatch chemicals to the brain’s master clock. There, they trigger about 20,000 neurons, or nerve cells, which “talk” to the rest of the body.

Those neurons tell the body when to release hormones. Hormones relay chemical instructions to distant parts of the body. These chemical messengers turn on activities at precise times and in specific cells. They say, for example: It’s time to eat. Or it’s time to wake up. Or it’s time to be really alert. But the clock — and the cycles it regulates — can become confused when people travel across many time zones, triggering jet lag.

Scientists once thought that the brain’s central clock was the body’s only one — that it alone directed every tissue in the body. But in the past 15 years, researchers have found the real story is much more complex.

“We’re finding that each little cell [in the body] has its own clock,” explains Sarah Forbes-Robertson at Swansea University in Wales. But scientists do not yet have a complete picture of all those clocks or how they work together.

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