In brief
Light is the strongest signal for your body clock. The brain's master clock uses the light your eyes receive to time melatonin and sleepiness.
- Shift work and jet lag alter the normal sleep-wake cycle.
- Melatonin levels rise when it is dark.
How it works
Light has perhaps the greatest influence on your body clock. The master clock in the brain controls melatonin based on how much light the eyes receive, and in the evening it tells the brain to make more melatonin, causing sleepiness.
Light at night can lower the amount of melatonin you make, and blue light from screens can reduce or delay it in the evening. Spending a few minutes outdoors on most days also helps you get vitamin D, but use sun protection when the UV Index is 3 or above.
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Morning daylight helps control
Light has perhaps the greatest influence on the body clock, which controls melatonin based on how much light the eyes receive.
Sources: National Institute of Neurological Disorders and Stroke (NIH), Brain basics: understanding sleep; National Institute of General Medical Sciences (NIH), Circadian rhythms
Sources
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Summarised from the NIH, healthdirect and Cancer Council. Sources read on 13 October 2026. General education only, not medical advice. Spotted a mistake? Please tell us through the HuBu-Intel contact page.