Circadian rhythms are most associated with the sleep/wake cycle of the body. This is a vital circadian rhythm, but it’s not the only one that affects your health. We’ll also discuss the importance of circadian rhythms for digestion, absorption, metabolism, exercise and fat-burning.

What is a Circadian Rhythm? 

A circadian rhythm is a natural bodily process that happens roughly every 24-hours. It’s a cycle, like sleeping & waking, or eating & digesting, that completes & restarts on a daily basis. Our circadian rhythms are tied to the earth’s rotation and light/dark cycle, but they are also encoded in our genes. [1]

For example, people are naturally diurnal animals, meaning we’re awake during the day and asleep when it’s dark. That’s no accident. It’s how we’re designed, like how mice are designed to be nocturnal mammals that sleep during the day and forage at night. [2]

Another example of an internal process operating on a circadian rhythm is the human digestive system. Our digestive system cannot create and burn fat at the same time, so our cells alternate according to a circadian rhythm. When we’re awake and actively taking in calories, our digestive system breaks down food and creates stored fat. When we sleep, our stored fat keeps our bodies fueled. [1]

Going against your natural rhythms and not establishing routines can have systemic effects, like sleep disturbances and metabolic slowdowns. Poor sleep can affect hormone production as well as your ability to recover from injury and overcome inflammation. When your body and cells are out of balance, you perform less efficiently across the board. [1]

Scientists are still researching and learning about our circadian rhythms. In 2017, three scientists were awarded the Nobel Prize for their research into the circadian data encoded in fruit fly genes. This showed that the cycles organisms live by cannot be explained as a mere reaction. Circadian rhythms are ingrained at a deep, cellular level. [3]

3 of the Body’s Core Circadian Rhythms

Dr. Satchin Panda of the Salt institute is the author of the book The Circadian Code, which discusses circadian biology in much greater detail. Dr. Panda writes about three “core rhythms” that our body aligns with. 

Circadian Rhythm and Restful Sleep 

Our bodies work best when we’re getting restful sleep every day. But even if you’re getting 7-8 hours, you may not be aligned with your body’s optimal sleep cycle. 

In the absence of technology like artificial lighting and screens, the human body will typically sync with the planet’s day/night cycle: rising with the sun and sleeping when it’s dark. We’ve come a long way from those origins, but our bodies are still designed to work with an abundance of light during the day, and restful sleep when it’s dark at night. If you sleep from 3am to Noon every day, you may get enough sleep, but the quality of your sleep will likely not be as good. [4] 

To test this theory, a researcher in 2013 brought a group of self-proclaimed “night owls” on a camping trip. Beforehand, the night owls monitored and recorded their sleep patterns. They even produced saliva samples to show when their body produced the most melatonin, the hormone that helps regulate sleep. Those results showed their bodies did indeed have a delayed onset for melatonin, producing the hormone around 10 pm, hours later than most people. [4]

Then, the night owls camped for 2 days with no external stimulation like bright lights, coffee, or screens. They were tested again and their saliva showed after just two days in the wilderness, they were producing melatonin earlier in the evening, so they could fall asleep sooner. [4,5] This is just one example of how most people’s bodies work best when they sleep and wake closer to the natural pattern of the sun and the earth.

Circadian Rhythm and Eating

The stomach, liver, pancreas, and other key internal organs work best when our eating aligns with our circadian rhythm. Following a routine, or schedule, with your eating that allows your body to operate along the same patterns is more efficient for your digestion, nutrient absorption, and overall metabolism. Limiting your food intake to a specific window of time is also ideal. [4] 

According to Dr. Satchin Panda, our body has an 8 to 10 hour window for optimal food intake that begins when we take our first bite in the morning. Digestion of even a small bite of food takes hours, and efficiency slows dramatically once we’re outside that window. There’s a set window of time because our internal organs follow circadian rhythms to do their jobs, like processing food and liquids. [4]

Our cells cannot store and break up body fat at the same time. Whenever we eat, the fat-making mechanisms in our body kick into action. When we consume fats outside of that 8 to 10 hour window, we are asking our bodies to go back into fat-storing mode, thwarting their normal schedule of burning fat. If you only eat in the same 8-10 hour window every day, your organs and body can get in a better rhythm of digestion and fat production. Ultimately, that can help you burn more fat and look and feel better. [4]

Circadian Rhythm, Exercise, and Recovery

It might seem obvious, but our bodies are made to be active while we’re awake. Physical activity during the day, in line with our sleep and digestion cycles, is a key part of a balanced, healthy lifestyle. Exercise has too many health benefits to list, but it’s most beneficial when you get your main activity done during the day, not late at night. [4] 

Our bodies are programmed to shift into a night mode after the sun goes down. Heart rate and breathing slow, body temperature lowers, and we usually go to sleep. When you’re exercising after dark, your body has to work hard to make and use all that energy, and then has to start the recovery process late in the day. This can clash with your sleep cycle and the circadian rhythm of digestion & eating, making your recovery and fat burning less efficient. [4]

How Does Light Affect our Circadian Rhythms?

Light is central to our internal patterns and biology. Our circadian rhythms are built around the sun and the earth’s rotation cycle. As the length of daylight changes throughout the year, our circadian rhythms adjust as well. [5]

Light intake is one of the most important factors in sleep and circadian rhythm. The brain interprets light as a sign of when to be asleep and awake. Your circadian rhythm is designed around the sun, but the bright lights and screens of modern life can knock the body’s natural signals and rhythms out of whack. If you stare at a bright screen at midnight, your body may be tired, but your brain is getting the message that it’s time to be awake. Too much bright, artificial light after dark can make it much more difficult to sleep and establish a healthy wake/rise schedule. [5]

Sleep hormones and light: Two hormones that regulate the sleep cycle are directly affected by light. Cortisol, a steroid hormone produced by your adrenal glands, helps wake us up and keep us going. Cortisol levels tend to be highest in the mornings and lowest when we’re in our deepest sleep, typically around 3-4am. Bright artificial light can stimulate cortisol levels that keep you awake, as documented in clinical studies. [6]



The hormone melatonin is a counterpart to cortisol. Produced by the pineal gland, melatonin helps you fall asleep and stay asleep. Your body usually starts producing melatonin in the early evening, when you’re starting to wind down and get closer to bedtime. Bright light, especially bright blue light from phones and computers, has been shown to disrupt melatonin production. [4,5,7]

Sources and References:

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[1] Sleep Foundation. Circadian Rhythm.

[2] Division of Sleep Medicine at Harvard Medical School. Individual Variation and the Genetics of Sleep.

[3] The Nobel Prize. 2017 Press Release.

[4] Book: Satchin Panda, PhD. The Circadian Code: lose weight, supercharge your energy, and transform your health from morning to midnight. Rodale Books. Jun 12, 2018 | ISBN 9781635652437

[5] Duffy J, Wright K. Entrainment of the Human Circadian System by Light. Journal of Biological Rhythms. 2005 Aug.[6] Jung C, Khalsa S, et al. Acute Effects of Bright Light Exposure on Cortisol Levels. Journal of Biological Rhythms. 2010 Jun.

[7] Harvard Health. Blue light has a dark side. Published 2012 May, updated 2020 July.