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, even overcast: many times brighter than typical indoor lighting
- Most useful single change: Get outside within a couple of hours of waking.
- Commonest mistake: Varying your wake time on 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, even overcast | many times brighter than typical indoor lighting |
| Teen body clocks | shift later in adolescence |
| AAP advice on school start times | middle and high schools at 8:30 a.m. or later |
| Melatonin | released as darkness falls, suppressed by light |
In one paragraph
Light timing sets it, which makes morning daylight one of the most powerful and least used sleep tools 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 on weekends, which works like jet lag
- Bright light late in the evening
- Expecting a bedtime change alone to move the clock
Related topics
- Teen Sleep: What Published Guidance Supports (Teen Health)
- What the Guidance Really Says About Shift Work and Health (Workplace Health)
- The Evidence on Screen Time and Mood, Without the Hype (Mental Health)
- What the Guidance Really Says About Sleep and the Brain (Brain Health)
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
- Sleep: The Complete Guide
- The Sleep Environment: What Published Guidance Supports
- Sleep and Physical Health: What the Evidence Actually Says
When to speak to a doctor
See your doctor if you consistently cannot sleep and wake at socially normal times despite trying, which may be a circadian rhythm sleep disorder, and mention any pattern of low mood in winter, as there are treatments. If you have thoughts of harming yourself, call or text 988, and call 911 if you are in immediate danger.
Where this comes from
- NIH — Circadian Rhythms
- American Academy of Pediatrics — School Start Times for Adolescents
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, summarized or generalized 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 your doctor, 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 911.
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