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What do people get wrong about the body clock?

This answer is a draft. It stays unpublished until legal review and source verification are complete.

"Body clock" is a familiar phrase in every language. The key fact — that it does not run on exactly 24 hours — is surprisingly little known. If you have recently moved to Japan and are still adjusting, that one fact explains a lot.

The body clock in numbers

• Measured under strictly controlled experimental conditions, the human body clock is generally described as having a cycle of about 24.2 hours. It used to be quoted as "25 hours", but more precise measurement gives a value closer to 24.
• Longer than 24 hours means that left alone, it drifts later by ten-odd minutes every day.
• The main signal that resets that drift is light: exposure to light as strong as sunlight tells the body clock what time it is.

[Verify] "About 24.2 hours" is the value given in the Pharmaceutical Society of Japan's glossary and similar sources. Confirm it against a primary source (academic society or research institute) before publication.

Misunderstanding 1｜It runs on exactly 24 hours

It is 24.2. That is why it needs adjusting every day.

Misunderstanding 2｜Light is the only thing that sets it

Meal times and the timing of activity are generally described as playing a part as well. Whether or not you skip breakfast is one of the factors.

Misunderstanding 3｜You can catch up by sleeping in at the weekend

If your waking time shifts a long way at weekends only, the clock shifts with it. Keeping the gap with weekdays small makes it easier to manage. The Ministry of Health, Labour and Welfare's *Sleep Guide for Health Promotion 2023* also stresses a regular daily rhythm.

Misunderstanding 4｜Night owls can be fixed, and should be

Morning types and evening types are generally described as individual differences. The assumption that "everyone should become a morning person" is not accurate.

Misunderstanding 5｜Drift has nothing to do with health

If a drift continues to the point where it disrupts your daily life, do not try to deal with it on your own — see a doctor.

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Sources

• Pharmaceutical Society of Japan [公益社団法人 日本薬学会], "Circadian rhythm" [概日リズム] (glossary of pharmaceutical terms)

https://www.pharm.or.jp/words/word00678.html

• Ministry of Health, Labour and Welfare (MHLW) e-Health Net, "Good sleep and lifestyle habits" [快眠と生活習慣]

https://www.e-healthnet.mhlw.go.jp/information/heart/k-02-001.html

• MHLW, *Sleep Guide for Health Promotion 2023* [健康づくりのための睡眠ガイド2023]

https://www.mhlw.go.jp/stf/seisakunitsuite/bunya/kenkou_iryou/kenkou/suimin/index.html

⚠️ Editorial note: the source URLs above were identified by search. Our working environment cannot reach go.jp domains, so the contents have not been checked against the text. Verify every source before publication and record who checked it and when.

About this article

What we share here are ideas and everyday practices passed down among people living in Japan, together with general information drawn from public sources. They are not official recommendations, and they are not professional instruction. Practices vary between households and regions.

This is general information. Results vary from person to person and depend on your own conditions and surroundings.

Please decide for yourself, using your own judgement, whether any of this suits your situation. We cannot accept responsibility for any damage, loss or injury arising from following this article.

This article is not meant to diagnose or manage any specific medical condition. If you are worried about your health, see a doctor rather than relying on your own judgement.

Sources

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