Light: the daily reset button

Article 7 min

You will be able to: Say how morning and evening light move the body clock and melatonin, and use light on purpose.

Your body clock runs a little longer than 24 hours, so it needs a daily nudge to stay in time with the world. Light and dark have the biggest influence on it.

How light reaches the clock

The master clock gets information about light straight from your eyes. Besides the cells you see with, the back of the eye has a small set of light-sensitive nerve cells that respond to brightness and pass that signal on. Because of this, light can reset the clock even when you are not paying attention to what you see.

What the clock does with that light depends on when it arrives:

Light in the morning, soon after your usual wake-up time, tends to move the clock earlier. You get sleepy a little earlier in the evening and wake more easily.
Light late in the evening and at night tends to move the clock later. You feel alert later and find it harder to get up.

This is why the same lamp can be helpful at 7:00 and unhelpful at 23:00.

Melatonin: the darkness signal

Melatonin is a hormone made by the pineal gland in the brain, in response to darkness. Its levels rise in the evening as it gets dark and stay up through your biological night. It helps you feel sleepy when it gets dark and helps time your daily rhythms, so a good way to think of it is as a darkness signal, not an off switch.

Light at night can block melatonin. That effect is not limited to very bright lights.

What about screens?

Phones, tablets and e-readers give off light too, much of it at the blue end of the spectrum, which the clock's light-sensing cells respond to strongly. The NHS advises not using devices right before bed, because the blue light makes you more awake.

In a lab study, 12 young adults read on a tablet for about four hours before bed on five evenings in a row, and on five other evenings read a printed book in the same dim room. After the tablet evenings, they took longer to fall asleep, felt less sleepy in the evening, made less melatonin, their body clock ran later, and they were less alert the next morning.

An honest caveat: it was a small study with long reading sessions in an otherwise very dim room, so it does not tell us exactly how much a shorter, ordinary evening on a phone matters. Screens can also keep you awake simply because what is on them is interesting. Either way, putting the phone down before bed is cheap to try.

Experts put numbers on light

In 2022 a group of light and sleep researchers published recommendations for indoor light. In short: plenty of light during the day, using daylight first where you can; a dim evening, starting at least 3 hours before bed; and a bedroom that is as dark as possible while you sleep. Their limits are set in a special unit that measures how strongly light acts on the body clock. They note that many warm-white home LED lights can stay within the evening limit if they are kept fairly dim (their example is about 30 lux, which is dimmer than typical room lighting), while bright, bluish-white light is more likely to go over it.

Source: Brown et al. (2022), Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults, PLoS Biology 20(3): e3001571, PubMed 35298459.

Sort it: does this light help or hurt your sleep?

Using light on purpose

These are practical suggestions built on the findings above, not medical rules:

Get outside within an hour of waking, even for 10 to 15 minutes. A walk to the shop or the station counts.
Work near a window in the morning if you can.
Dim the evening: warm, low lamps instead of bright ceiling lights for the last few hours.
Keep the bedroom dark, and the phone out of reach.

Quick check (3 questions)

Sources

NIGMS (NIH), Circadian rhythms (light and dark as the biggest influence; the SCN controls melatonin based on the light the eyes receive)
NINDS, Brain basics: understanding sleep (the SCN gets light information directly from the eyes; melatonin from the pineal gland)
NCCIH, Melatonin: what you need to know (made in response to darkness; light at night can block it)
Khalsa et al. (2003), A phase response curve to single bright light pulses in human subjects, Journal of Physiology 549(3): 945-52, PubMed 12717008
Gooley et al. (2011), J Clin Endocrinol Metab 96(3): E463-72, PubMed 21193540
Chang et al. (2015), Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness, PNAS 112(4): 1232-7, PubMed 25535358
Brown et al. (2022), PLoS Biology 20(3): e3001571, PubMed 35298459 (light-sensitive cells in the eye; the day, evening and night limits; indoor light much dimmer than outdoors)
Wright et al. (2013), Entrainment of the human circadian clock to the natural light-dark cycle, Current Biology 23(16): 1554-8, PubMed 23910656
NHS, Insomnia (dark, quiet bedroom; no devices right before bed)
NHS, Seasonal affective disorder: treatment (speak to a GP before light therapy)
NHS, Melatonin (prescription only in the UK)

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