The Circadian Rhythm: What Is It and Why Is It Important?

Dr. Satchin Panda is a professor in the Regulatory Biology Department at the Salk Institute for Biological Studies in La Jolla, California, and founding executive member of the Center for Circadian Biology at the University of California, San Diego. He is a leading expert in the field of circadian rhythm research and the author of The Circadian Code: Lose Weight, Supercharge Your Energy, and Transform Your Health from Morning to Midnight.

Dr. Panda’s work deals with the timing of food and it’s relationship with our biological clocks governed by circadian rhythm and also the circadian rhythm in general. He is a Pew Scholar and a recipient of The Julie Martin Mid-Career Award in Aging Research.


Recovery is the glue that keeps an athlete from falling apart. However, sometimes it seems that even when we do the right workout, eat the right foods, drink the right smoothies, roll all the right muscles, and sleep the right number of hours, we still don’t wake up the next day feeling as refreshed as we expect.

Are we getting too old? Are we pushing too hard? Are we just not cut out for sports?

Naturally, we become lifestyle detectives and think back on what we could do better, but what we don’t realize is that we might, in fact, have done all the right things — just at the wrong time.

Dr. Satchin Panda, author of The Circadian Code, argues that when it comes to human health and performance, timing is everything. Nearly every aspect of our health, he says, is affected by our relationship to a daily, 24-hour cycle called the circadian rhythm. By getting to know our circadian rhythms, we can adjust the timing of our daily activities in simple ways to enhance performance.

What is the Circadian Rhythm?

The word “circadian” comes from the Latin circa, which means “approximately,” and diem, which means “day.” The term “circadian rhythm” is used to talk about all the body processes that happen on a regular, near-24-hour interval — in other words, “approximately every day.”

According to Dr. Panda, who has spent more than 21 years studying circadian rhythms and works at the Salk Institute for Biological Science, every gene in the human genome turns on and off like a switch in response to a daily timeline. This affects all cellular activity — from normal, cell-specific functions to repair, regeneration, and growth. For us, the athletes made up of these cells, this means that our health, cognition, and performance are affected not only by what we do, but when we do it.

How “approximate” is this rhythm?

Despite being called an “approximate” daily rhythm, the human circadian rhythm is actually surprisingly precise. In the 1950s, a researcher wanted to figure out exactly how precise the body clock was, so he secluded himself in a cave in the Andes mountains equipped only with food, candles, water, and an old-fashioned phone. His task was simple: call his research partner once when he woke up and once when he was feeling tired enough to fall asleep. Even though the researcher was in complete darkness and had no idea what time it was, the calls came at the expected times every day.

What your body wants to do every day

The human circadian rhythm is centered around three things: light, breakfast, and physical activity — in that order. In the morning, while we sleep, the body tapers down its production of melatonin, the hormone that makes us feel sleepy. Breathing, heart rate, and core temperature gradually increase until we finally wake up, open our eyes, and take in the first light of the day — historically, the blue light of the sky.

This is the first crucial moment.

This blue light resets the body’s master clock (also known as the suprachiasmatic nucleus or SCN), which synchronizes the clocks in the pineal gland, pituitary gland, adrenal gland, and the rest of the hypothalamus. It’s the head coach of all the brain’s timing and sets the pace of the entire body.

One result of this is that we feel hungry; the master clock in the brain tells the stomach to secrete ghrelin (the hunger hormone) and the other organs to get ready to digest food. The first bite of food we swallow, which we’ll call “Bite Zero,” then triggers the second major event of the human body clock, this time in the liver.

For the next 8 to 10 hours after Bite Zero, the internal organs are primed and ready to process any food that comes their way. This typically comes in the form of a few meals interspersed throughout the day. During this window, our liver and muscles start what Dr. Panda calls a “fat-making program” to turn food into stored energy in the form of glycogen and fats. But this opportunity doesn’t last forever.

After the window of eating opportunity, or a few hours after we ingest our last calories of the day (which we can call Bite N), the organs naturally transition to “recovery mode.” Digestion is a cellular workout, and the internal organs need time to rest and recover so they can do it all again the next day. Provided the stomach receives no new food after dinner, the liver and muscles start literally burning the midnight oil — stored fat — and using that energy to sustain and repair the body overnight.

Finally, as the natural light of day fades and our core temperature drops, the body stops suppressing the production of melatonin. Melatonin accumulates in the blood until we feel tired, go to bed, and fall asleep. Then, throughout the night, our cells grow, repair themselves, and flush out toxins so that we wake up feeling refreshed.

This is, at least, what our bodies want to do every day. And it’s probably what would happen if you spent your days camping in Maasai Mara National Reserve, where night is night and day is day. Unfortunately, things aren’t so simple in modern cities. Unnatural light, 24-7 access to food, and a lack of physical activity derail this rhythm, and our bodies pay the price.

This, Dr. Panda says, is the biggest obstacle to health in the 21st Century.

What happens when you get away from your circadian rhythm?

To reiterate, the circadian rhythm affects essential processes in every aspect of human health: attention, memory, emotions, hormone secretion, digestion, metabolism, cell repair, and even athletic performance. It is not surprising, then, that a disrupted circadian rhythm increases a person’s risk of numerous health problems. As Dr. Panda puts it, “every system in your body starts to malfunction” when we deny our bodies the routine they are programmed to follow, whether by staying up late at night or waking up several hours later on the weekends. In fact, studies of mice have shown that just a few weeks of interrupted sleep can make mice so frail and prone to infection that they will die if left untreated.

For humans, the consequences are just as dire but more complex. Not only do we feel physically ill when we lose sleep, but we suffer cognitive impairment as well. A few days of reduced sleep, says Dr. Panda, can distort our appetite, cause us to crave junk food, and snip our attention spans. This makes us less likely to make healthy choices on a day-to-day basis. As a result, long term circadian disruption can lead to more serious health problems like the following:

  • Digestive problems
  • Obesity
  • Diabetes
  • Cardiovascular problems
  • Slowed recovery from illness or injury

The circadian rhythm directly affects athletic performance.

Athletes face special consequences for disrupting their circadian rhythms. For example, a disrupted rhythm stifles our body’s natural repair process for cartilage, which compounds the effects of aging on our joints. It throws a wrench into the bone-making process, too, which can lead to weaker bones or even fractures. Muscle fibers, which depend on circadian genes for fuel and repair, also lose out when our circadian rhythms get off track.

All these things may indirectly affect your athletic performance, but the circadian rhythm has direct effects as well. According to Dr. Panda, athletic performance can vary “as much as 25 percent within a day,” with motor coordination and strength peaking in the late afternoon. In fact, a study of Monday night football games found that when West Coast teams flew to the East Coast to play, they had a significantly higher chance of beating the East Coast teams, provided they played within 48 hours of travel. This was because the bodies of the West Coast football players were playing at 6:00 p.m. Pacific Time; the East Coast team, however, was playing at 9:00 p.m. Eastern Time, well outside their window of peak performance.

How can you fix your circadian rhythm?

There are simple (not easy) steps that we can take to fix our circadian rhythms. Dr. Panda’s core solution involves a three-pronged prescription of blue light, food, and exercise, which we will cover next week in a follow-up blog. In the meantime, take a minute to answer the following questions about your daily routine.

  • What time do you wake up? Does this time change from day to day?
  • What time do you wake up on weekends?
  • When is your “Bite Zero,” and when is your latest snack of the day? (Here a “snack” is anything with calories.)
  • Do you feel well-rested when you get up in the morning?
  • How many hours of sleep do you get each night?
  • When do you go to bed? Does this time change from day to day?


Your performance is the result of every decision you make, and timing is a huge part of that.

TB12 is about doing what you love better and longer. That means following the body’s natural cues: cues to rest and recover, cues for basic behaviors like eating, drinking, and sleeping, and cues to keep going. To learn more about the ways you can hack your circadian rhythm for optimal performance, read Dr. Panda’s book The Circadian Code and subscribe to the TB12 Newsletter to receive the latest workouts, recipes, and news.