Circadian Rhythm 101

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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 daily. Our circadian rhythms are tied to the earth’s rotation and light/dark cycle, but they are also encoded in our genes.

We are naturally diurnal animals, meaning we’re awake during the day and asleep when dark. It’s how we evolved, like how mice are designed to be nocturnal mammals that sleep during the day and forage at night.

Another example of an internal process operating on a circadian rhythm is the human digestive system. Our digestive system cannot create and burn fat simultaneously, 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.

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.

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 & Sleep

Without artificial lighting and screens, we 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 5 am to 2 pm every day, you may get enough sleep, but the quality of your sleep will likely not be as good.

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.

Circadian Rhythm & Eating

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.

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.

Circadian Rhythm & Recovery

Our bodies are programmed to shift into a night mode after the sun goes down. Heart rate and breathing slow, body temperature lower, 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 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.

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.

Light & Circadian Rhythms

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.

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 3–4 am. Bright artificial light can stimulate cortisol levels that keep you awake, as documented in clinical studies.

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 disrupted melatonin production.



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