The circadian rhythm: the internal clock that regulates your energy, sleep and concentration

In short

The circadian rhythm is the biological cycle of about 24 hours that regulates sleep, alertness, body temperature and the release of several hormones, such as melatonin and cortisol. The main signal that matches it to the real day is light. Natural light in the morning helps it synchronise, while bright light in the evening can delay it. Consistent sleep hours, meals at regular times and darker evenings are the simplest habits for an aligned internal clock.

What the circadian rhythm is

"Circadian" comes from the Latin "circa diem", meaning "around a day". In the brain, a small area of the hypothalamus called the suprachiasmatic nucleus works as a central clock. It coordinates smaller clocks in the liver, muscles, heart and other organs.

Over 24 hours, the internal clock influences:

when you become sleepy and when you wake up;
body temperature, lower at night;
the release of melatonin in the evening;
the cortisol peak in the first hours after waking;
digestion and metabolism;
alertness and mental performance.

Research on the molecular mechanisms of this clock was awarded the Nobel Prize in Medicine in 2017.

Light: the main signal

The internal clock doesn't run at exactly 24 hours, so it needs daily signals to recalibrate. The strongest signal is the light that reaches the eyes.

morning light tells the brain that the day has begun and helps sleep come more easily in the evening;
bright light in the evening, including from screens and cool-toned bulbs, can delay the release of melatonin;
darkness signals that night is coming.

Outdoor light, even on a cloudy day, is usually much stronger than the light in an office. That's why a few minutes outside in the morning matter more than they seem.

In the morning, look for light. In the evening, look for dimness.

Chronotype: larks, owls and most of us

People differ in the time of day when they feel best:

morning chronotype: you fall asleep early and have energy from the first hours;
evening chronotype: you come alive late and find early mornings hard;
intermediate chronotype: most people.

Chronotype has a genetic component and changes with age: teenagers tend to be more "evening" types, and older people more "morning" types. You can't change it completely, but you can shift it a little through light and consistent hours.

When the internal clock is out of sync

jet lag, after flights across several time zones;
social jet lag, when at the weekend you go to bed and wake up much later than during the week;
shift work, especially at night;
evenings with screens and bright light until late.

The usual signs: tiredness in the morning, difficulty falling asleep, drops in energy at unusual times, disrupted appetite and weaker concentration.

Habits for an aligned circadian rhythm

Natural light in the first hour after waking

10–20 minutes outside or next to an open window. A short walk, coffee on the balcony or walking to the office is enough.

Consistent wake-up times

Your wake-up time is a stronger anchor than your bedtime. Try to keep it fairly consistent at the weekend too, with a difference of an hour at most.

Meals at regular times

Meals are also signals for the clocks in the organs. A very late, heavy dinner can disrupt sleep.

Darker evenings

After sunset, prefer warm, dimmer lights. Reduce screens in the last hour before bed or use night mode.

Movement during the day

Physical activity, especially in the first part of the day, supports a clear rhythm of activity and rest.

Coffee, in the morning

Caffeine can stay active for many hours. For most people, the last coffee should be drunk at least 8 hours before bed.

The rhythm of the day and work

For many people, alertness peaks in the second part of the morning, dips naturally in the early afternoon and has a second, smaller peak in the late afternoon. Use this information:

tasks that require thinking, in the morning;
meetings and routine tasks, in the early afternoon;
a short break or a walk when your energy dips.

Winter and light

In months with short days, many people feel less energy and a lower mood. Natural morning light becomes even more important. If sadness and lack of energy last for weeks and affect your everyday life, talk to a doctor or a psychologist.

How NeuroFit complements your routine

NeuroFit by Emmanuel CliniX, in Bucharest, doesn't replace natural light or sleep. It can, however, be a fixed hour in your week dedicated to recovery, which gives you rhythm.

Reset (taVNS, tPBM, CES): after periods of disrupted sleep, travel or long shifts;
Relax (CES, taVNS, neurofeedback): when the mind stays active in the evening.

Opening hours of Monday to Friday, 09:00 to 22:00, and Saturday, 10:00 to 16:00, let you find a consistent hour. The first experience costs 190 lei. Bookings: +40 790 099 070.

NeuroFit is brain wellness, not medical treatment.

Frequently asked questions

What is the circadian rhythm?

It's the biological cycle of about 24 hours that regulates sleep, alertness, body temperature and hormones such as melatonin and cortisol. It's coordinated by a central clock in the brain and synchronised mainly through light.

How do I reset my circadian rhythm?

Through natural light in the first hour after waking, a consistent wake-up time at the weekend too, meals at regular times, evenings with dimmer light and fewer screens before bed.

Why do I feel tired in the afternoon?

For many people, alertness dips naturally in the early afternoon, as part of the daily rhythm. Insufficient sleep and a heavy lunch can accentuate this dip.

What is social jet lag?

It's the difference between your sleep schedule during the week and at the weekend. When you go to bed and wake up much later at the weekend, the internal clock gets out of sync, and Monday morning feels harder.