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Circadian Rhythm Calculator

Created by Davide Borchia
Reviewed by Steven Wooding
Based on research by
Hirshkowitz M, Whiton K, Albert SM, Alessi C, Bruni O, DonCarlos L, Hazen N, Herman J, Katz ES, Kheirandish-Gozal L, Neubauer DN, O'Donnell AE, Ohayon M, Peever J, Rawding R, Sachdeva RC, Setters B, Vitiello MV, Ware JC, Adams Hillard PJ. National Sleep Foundation's sleep time duration recommendations: methodology and results summary.; Sleep Health; March 2015See 1 more source
Shen, X., Wu, Y. & Zhang, D. Nighttime sleep duration, 24-hour sleep duration and risk of all-cause mortality among adults: a meta-analysis of prospective cohort studies; Scientific Reports; February 2016
Last updated: Jan 18, 2024

The circadian rhythm calculator allows you to find the best time to head to sleep if you want to follow the biological rhythms of the human body. Keep reading our short article to learn:

  • What is the circadian rhythm?
  • How to use the circadian rhythm to calculate sleep hours.
  • And a bit of the science of sleeping.

What is a circadian rhythm?

The regular rotation of Earth around its axis in 24 hours tuned the biology of our planet according to the same time frame. Plants, animals, and fungi (the other two kingdoms, Protista and Monera, are only marginally affected by the alternation of day and night) grow, rest, feed, and generally live following the succession of dawn, daylight, twilight, and night.

Even you, immersed in an environment that only marginally resembles the one where our species evolved, feel the effects of this natural clock. Scientists call the change in biological function according to the moment of the day a circadian rhythm. The word circadian comes from Latin, a mixture of circa, "about", and dies, "day", a way to highlight the repetitive, 24-hour long period of such phenomena.

In average, natural conditions, your body would tune with the presence or absence of sunlight. Humans are diurnal animals. Thus, our body reacts to the presence of sunlight with a generally increased alertness due to hormonal and cognitive changes that happen early in the morning. In the evening, different changes suggest to your body that it's time to sleep.

In the absence of any external clue, studies proved that the human internal clock is tuned to about 24.1 hours. What happens if we disrupt the circadian rhythm?

The introduction and spread of artificial lightning and a general change in our lifestyles (night shifts, longer working days, faster communication, and a general detachment from the natural world) marked an increased distance between how we operate our bodies and how our bodies work.

Light sources with a high percentage of blue color, for example, can block the production of melatonin, a hormone that suggests our body goes to sleep. Exposure to such light sources is a prime factor in disrupting our sleep cycles. Interfering with the natural circadian rhythms causes a plethora of both physical and mental disruptions, for example:

  • Increased tendency to obesity;
  • Insomnia;
  • Increased risk of depression;
  • Increased occurrence of cardiovascular issues;

and many more effects, among which even a positive correlation with certain types of cancer.

How to take care of your sleep rhythms? One of the first things you can do is learn how to calculate the circadian rhythm to align with your sleep cycles. A sleep cycle is a biological subdivision of human sleep, and "completing" a sleep cycle is usually associated with increased sleep quality. In the next section, we'll see how to calculate circadian rhythm and sleep cycles.

How to calculate your circadian rhythm: calculate sleep cycles and much more

Your circadian rhythm best aligns with sunrise and sunset. So to calculate the best time for sleeping according to the circadian rhythm, find the local time of sunrise and sunset!

💡 To find the exact time of sunrise and sunset, visit our dedicated tools, the sunrise calculator and the sunset calculator.

A study on three pre-industrial societies showed that humans, in the absence of interference, tend to go to sleep between 2.5 and 4.5 hours after sunset. Naturally, the best time to wake up also matches the sunrise (sleeping from sundown to sunrise is not what we would do naturally, and our habit of going to sleep later in the night is assumed to be one of the origins of culture: how many stories were told in front of a fire?). Now that we set those two limits, we can calculate the best sleeping time according to the sleep cycles.

How to do so? Count backward from the sunrise time in 90-minutes intervals. You are good to go when you reach a time that falls in the ideal hours after sunset, considering the time you usually need to fall asleep!

🙋 Are you a short-sleeper? Or maybe a long-sleeper? As this study shows, genetics plays a fundamental role in your habits. However, regardless of your control over this, not respecting the circadian rhythms can lead to health issues. Don't neglect your sleep!

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How do I sleep following the circadian rhythm?

To sleep following the circadian rhythm:

  1. Try to go to sleep between 2 and a half and 4 and a half hours after sunset.

  2. Complete as many 90-minute sleep cycles as possible to wake up around sunrise.

  3. Reduce your exposure to artificial light after sunset, and in particular, avoid blue light.

Following these simple rules and basic sleep hygiene will increase the chances of waking up rested.

When should I go to sleep to wake up at 7:00, following circadian rhythms?

If you want to wake up with the sunrise at 7:00 AM, the best time to go to sleep is at around 11:15 PM. This time best aligns with the circadian rhythm, allowing you to go to bed between 2.5 and 4.5 hours after sunset and to complete 5 sleep cycles of 90 minutes each.

If you usually sleep for longer, the best time would be 9.45 PM. On the other hand, if you can deal with fewer hours of sleep, try to hit the bed at 12.45 AM.

Davide Borchia
What time should I go to sleep?
Time to fall asleep
Waking up time
7:00 AM

To wake up at the end of a sleep cycle, go to sleep at:
  - 9:45 PM (6 cycles, 9h of sleep) - recommended for long-sleepers,
  - 11:15 PM (5 cycles, 7h30m of sleep) - recommended for average-sleepers,
  - 12:45 AM (4 cycles, 6h of sleep) - recommended for short-sleepers,
  - 2:15 AM (3 cycles, 4h30m of sleep),
  - 3:45 AM (2 cycles, 3h of sleep),
  - 5:15 AM (1 cycle, 1h30m of sleep).

If you go to sleep NOW, you should wake up at:
  - 1:49 PM (6 cycles, 9h of sleep) - recommended for long-sleepers,
  - 12:19 PM (5 cycles, 7h30m of sleep) - recommended for average-sleepers,
  - 10:49 AM (4 cycles, 6h of sleep) - recommended for short-sleepers,
  - 9:19 AM (3 cycles, 4h30m of sleep),
  - 7:49 AM (2 cycles, 3h of sleep),
  - 6:19 AM (1 cycle, 1h30m of sleep).
How dangerous is my sleep routine?
Night-time sleep duration
7 hours
Increase in mortality risk
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