The Body Clock: The Quick Reference
The Body Clock: the whole thing in five minutes, for when you want the numbers and nothing else.
The roughly 24-hour internal rhythm that sets when you feel alert and when you feel sleepy.
Key takeaways
- Strongest signal: light, particularly in the morning
- Outdoor light, overcast day: typically 1,000-10,000 lux
- Most useful single change: Get outside within a couple of hours of waking.
- Commonest mistake: Varying your wake time at weekends, which works like jet lag.
- See a doctor if any of the signs under "When to speak to a doctor" below apply to you.
The numbers, in one place
| Measure | Figure |
|---|---|
| Strongest signal | light, particularly in the morning |
| Outdoor light, overcast day | typically 1,000-10,000 lux |
| Typical indoor office lighting | around 300-500 lux |
| Adolescent circadian shift | 1-3 hours later |
| Melatonin | released as darkness falls, suppressed by light |
In one paragraph
Light timing sets it, which makes morning daylight the most powerful and least used sleep intervention available.
Worth doing
- Get outside within a couple of hours of waking
- Keep the wake time fixed, which anchors everything else
- Dim the lights in the evening rather than only avoiding screens
- Shift the schedule gradually, by 15-30 minutes a day
Worth avoiding
- Varying your wake time at weekends, which works like jet lag
- Bright light late in the evening
- Expecting a bedtime change alone to move the clock
Where to go next
- Understanding The Body Clock: A Plain-English Explainer
- The Body Clock: The Practical To-Do List
- The Body Clock — the reference entry
- The Sleep Environment: What Published Guidance Supports
- Sleep and Physical Health: What the Evidence Actually Says
When to speak to a doctor
See a GP if you consistently cannot sleep and wake at socially normal times despite trying, which may be a circadian rhythm sleep disorder rather than insomnia. Seasonal patterns of low mood in winter are worth mentioning, as there are treatments.
Where this comes from
- NHS — Sleep and tiredness
- The Sleep Charity — sleep advice for adults
How this page was put together
Every figure, recommendation and warning above is taken from the guidance named in Where this comes from. Nothing on this page is inferred, summarised or generalised beyond what those bodies publish: where they give a number, it is their number, and where they are silent, so are we. Pages are rebuilt whenever the underlying record changes. Sign-off by a named clinician, where it has happened, is shown at the top of the page along with the date.
Medical disclaimer
This article is general information, not medical advice. It cannot account for your own history, medication or circumstances. Always speak to a GP, pharmacist or another qualified health professional before making changes that affect your treatment, and never delay seeking medical advice because of something you have read here. In an emergency, call 999.
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