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In Sync with Circadian Rhythms

We rely on the day-night cycle known as a circadian rhythm to keep our system running smoothly.

Circadian rhythm, morning, waking, young woman

A circadian rhythm syncs with our brain's master clock to create healthy patterns for sleeping, eating, and activity.

© iStockphoto.com/blackCAT

Setting the Master Clock

All living organisms rely on a master clock to coordinate the biorhythms they need to stay healthy.

For humans, it starts with a central control system in our brains. Our master clock is made up of about 20,000 neurons that form the suprachiasmatic nucleus, or SCN for short. This highly specialized area acts on light cues coming directly from our eyes.

These light signals trigger special cells to go to work and create proteins that establish a roughly 24-hour “circadian” rhythm in our bodies, which allows us to sleep, eat, and expend energy at the right time. Circadian is a portmanteau of two Latin words: “circa,” meaning approximately, and “dies” meaning “day.”

Temperature, exercise, and social activity can affect our master clock too, but it’s clear that light has the most powerful influence.

Circadian rhythm, Grandfather, kids, play

Our body clock depends on a circadian rhythm to help us expend energy at the right time.

© iStockphoto.com/supersizer

To Sleep, Perchance to Dream

Deep, restorative sleep and even dreaming are important for our health and well-being. To help us fall asleep, circadian rhythms trigger the production of melatonin —the hormone that makes us drowsy before bed.

At night, our optic nerves send information to the brain’s SCN area, letting it know that there is significantly less light, and this cue sets off the release of melatonin to start our bodies preparing for sleep.

When your circadian clock is thrown off, like when you change time zones and experience jet lag, or are engaged in shift work, the flow of melatonin gets interrupted, and you can suffer from circadian rhythm sleep disorders like insomnia.

But the role these cycles play goes far beyond establishing typical sleep patterns. Circadian rhythms are at work in many important body processes.

Timing is Everything

Science is getting a better understanding of just how widespread internal clocks like circadian rhythms are in our systems. Research has shown there is some kind of timing structure built into nearly every tissue and organ in our bodies.

The way a circadian rhythm works with other biological timers is a fascinating area of research. For example, when we are anticipating a meal, our digestive system uses cues from circadian rhythms to create proteins that enable us to draw the nutrients out of our food.

A study published on Gastrojournal.org in 2000 found data that helped expand our understanding of that process. These findings suggested that the ability to keep time may not be confined to the brain, but may also belong to individual cells within the digestive system itself. This may mean other organs have a way to keep time independently of the SCN.

Rising to the Need

Another area where circadian rhythms are important is in our endocrine system, which regulates the hormones we need to match our usual daily energy expenditure.

Over the course of a 24–hour period, our endocrine system uses a circadian rhythm to anticipate when we will need to be in activity, eat, fast, and rest, secreting hormones like cortisol to help us find the energy to perform important tasks.

We also need to heat up and cool down to fit our schedules. Thanks to regular cues from our circadian rhythms, our body temperature cools a few hours before we wake up and heats up about an hour before drifting off to sleep. Usually, there is about a 1 ° C difference in your core temperature between its lowest and highest points.

And circadian rhythms are found in life forms other than humans. In the natural world, flowers use internal timing mechanisms to open and close at the right time, nocturnal animals know the best time to leave their shelters, and there is evidence of circadian rhythms at work in the cycles of fungi, mice, fruit flies, and plants.