Circadian Rhythm: What It Is, How It Works, and How to Regulate It

We all have an internal clock that regulates when we wake up and when we sleep. It is called the circadian rhythm and it follows a 24-hour cycle (approximately). In addition to sleep, it regulates other processes involving hormones, body temperature, digestion, etc.

In this article, we explain what the circadian rhythm is, how it works, and what happens when it becomes dysregulated. Plus, we share hacks to help you “reset” it so you can sleep better and improve your energy and mental well-being.

What is the circadian rhythm?

The circadian rhythm is, basically, our biological clock. It is the rhythm our body follows based on a cycle of approximately 24 hours, and it tells us when to sleep, when to wake up, and when to be active.

Although we tend to associate it solely with sleep, the circadian rhythm is involved in many other processes in the body:

Hormone production: influences the release of cortisol, melatonin, insulin, and growth hormones at specific times of the day.
Body temperature: decreases core body temperature at night to prepare for sleep and increases it in the morning to promote alertness.
Metabolism and digestion: influences how the body processes food and regulates glucose.
Heart rate and blood pressure: blood pressure decreases during nighttime rest and cardiovascular activity increases upon waking.
Immune function: regulates inflammatory responses and immune cell activity according to the time of day.

In other words, the circadian rhythm works as a coordination system that helps different processes in the body take place at the right time of day.

How does it work?

Our circadian rhythm is connected to an internal clock in your brain, the “master clock.” What primarily “controls” this clock is light.

When light enters the eyes, it stimulates certain cells that signal to the “master clock” that it is daytime and that it is time to start releasing cortisol: the hormone that tells us “it’s time to activate.”

At sunset, as light begins to fade, the brain understands that the 24-hour cycle is entering its final stage and that it is time to start producing melatonin: the hormone that promotes sleep.

Although the internal clock responds primarily to light, temperature also acts as another signal. According to Andrew Huberman, light is the trigger and temperature is the determinant (the effector).

For example, after exposure to cold, the body starts working to restore its core temperature. And here’s the key: increases in body temperature tend to make us more alert.

What happens when the circadian rhythm is out of sync?

When our biological clock loses synchronization with the light-dark cycle, disruptions in sleep and other physiological processes can occur:

Sleep problems. This doesn’t just mean sleeping poorly or feeling tired during the day. Sleep is one of the most important physiological processes for health: it is when cellular repair takes place, the brain processes information, the immune system is strengthened, the heart rests, etc.
Cognitive changes: lack of sleep and circadian misalignment can affect attention, memory, learning, and decision-making.
Changes in mood: sleeping too little or at misaligned times can affect emotional regulation and the ability to manage stress.
Effects on metabolic health: sustained circadian disruptions have been associated with changes in metabolic processes and an increased risk of various health problems.

Astronauts and the circadian rhythm

This is a super interesting study that analyzes the effect of long-duration spaceflight on astronauts’ circadian rhythms.

Factors such as microgravity, radiation, and changes in light-dark cycles can disrupt sleep and dysregulate the biological clock. This combination, along with sleep deprivation and long working hours, can affect key cognitive functions and increase the risk of errors during mission-critical tasks.

The example of astronauts highlights something interesting: keeping our biological clock synchronized is not only about sleeping well, but also about performing and functioning properly during the day.

Tips and hacks to regulate your circadian rhythm

The good news is that there are simple habits that can help you synchronize your biological clock.

Daylight, darkness at night

One of the simplest and most effective tools is to expose yourself to natural light in the morning.

Dr. Samer Hattar recommends 10 to 30 minutes of morning natural light exposure per day. Even when it is cloudy, outdoor light is still much more intense than typical indoor lighting.

What about the afternoon or evening? The idea is to do the opposite: gradually reduce exposure to bright light as bedtime approaches. Dim light is your best ally. Soft red light can also be a useful tool since it provides illumination without activating the brain and disrupting our internal clock.

Screens at night? Your worst enemy. It’s like pointing a flashlight directly into your eyes. And as we’ve seen, light is the strongest signal for the master clock. So, if you can avoid screens altogether at night, even better. Otherwise, you can do what Dr. Hattar does: if you need to use your phone, keep it brief and tilt it so that it is not directly facing your eyes (so the blue light isn’t entering your eyes directly).

Routine

The brain is a creature of habit… What it likes most: consistency.

Try to go to bed and wake up at approximately the same time every day. Maintaining relatively consistent schedules helps provide predictable signals to your biological clock.

The same applies to meals. As we mentioned, the circadian rhythm also plays a role in digestion and eating behavior. Maintaining some regularity in meal times can help keep these processes synchronized. In addition, avoiding very large meals right before bed can promote better sleep.

Temperature

Core body temperature follows a circadian pattern: it decreases during the night and reaches its lowest levels during the early hours of the morning. Therefore, a cool bedroom can support this natural process and promote sleep.

There is no perfect temperature that works for everyone, but several experts recommend a temperature around 20-18 °C.

Other ways to use temperature to your advantage:

Cold-plunging in the morning: exposure to cold promotes the release of adrenaline and dopamine, acting as an activation signal for the body and brain.
Sauna in the afternoon or evening: heat exposure promotes relaxation and, when done sufficiently ahead of bedtime, can support the physiological processes associated with the subsequent decrease in body temperature.

The key is not only which tool you use, but when. Light, temperature, food, and sleep all act as signals that help keep our internal clock synchronized.