The guide

Melatonin and light: what lamps, screens and daylight do

By GyroscopeUpdated November 14, 20269 sources

Light at your eyes stops your own melatonin and moves your body clock, and it does both more strongly than any tablet. In a 2011 study, ordinary room light in the evening made melatonin start later in 99 percent of people. Light in the morning moves your clock earlier, and light in the evening moves it later.

Melatonin is your body’s message that it’s dark. Light is the message that it isn’t, and your brain trusts that one more.

Isn’t the phone the real problem?

The phone gets most of the blame, and it’s the easiest thing to point at. But the ceiling light, the TV and the bathroom mirror are light at your eyes too. In the 2011 study that found the 99 percent, the light was plain room light, under 200 lux, which is an ordinary evening at home.

People also expect the tablet to win. You take melatonin, you sit under bright lights, and you wonder why nothing happens. The tablet does put melatonin in your blood. The light keeps telling your brain’s clock it’s still day, holds back your own melatonin and pushes your clock later, which is the opposite of what you took the tablet for.

A simple way to picture it

Taking melatonin in a bright room is like turning on the heat with the windows open. The heater works. You just spend the evening fighting the cold air coming in, and the room never gets warm.

Close the windows first. Then a small heater is enough.

How did we learn light controls it?

For years it looked as if people were the exception. In other mammals, ordinary room light stopped night melatonin. Experiments in people had used room light and found no effect. In 1980 a team led by A.J. Lewy tried much brighter light on six people, and their night melatonin dropped. People respond to light like other mammals, but the early tests hadn’t used enough of it.

In 2011 a team at Brigham and Women’s Hospital and Harvard found that an ordinary evening of room light is enough to hold it back. 116 adults aged 18 to 30 lived in a research unit for at least five days. They spent the 8 hours before bed either in room light, under 200 lux, or in dim light, under 3 lux.

• Room light made melatonin start later in 99 percent of them.
• It shortened their melatonin night by about 90 minutes.
• Room light during their usual sleep hours cut melatonin by more than half in 85 percent of the tests.

How does light move your clock?

Your brain’s master clock, the suprachiasmatic nucleus, sets melatonin production by the amount of light your eyes take in. Light doesn’t just switch melatonin off for the moment. It also resets the clock, and like melatonin it pushes the clock in different directions at different hours.

The clearest map comes from a 2003 Harvard study. 21 healthy adults stayed awake in dim light, then got 6.7 hours of very bright light, about 5,000 to 10,000 lux, at a different hour for each person. Light before the body’s temperature low moved the clock later. Light after it moved the clock earlier. Light right at the low did nothing. From the biggest move later to the biggest move earlier, the curve spanned about 5 hours.

The temperature low usually comes about 3 to 4.5 hours before you wake, so for most people:

For scale, three days of 0.5 mg melatonin at its best hour moved the clock up to 1.5 hours in a lab. The two studies used different methods, but light is the bigger push.

What should you do with light?

1. Lamps only from your window to bed. No overhead lights. The study’s dim condition was very dim, under 3 lux, so dimmer is better, but cutting the ceiling light is the change people keep.
2. Keep the bedroom dark. Blackout curtains or an eye mask. Eastman and Burgess advise a dark bedroom, or a mask if you can’t get one.
3. Get outside within an hour of waking if you want to move your clock earlier.
4. Get daylight during the day. In a small Swedish study, people who’d had a bright day saw no effect from two hours on a tablet in the evening.
5. To move your clock later, do the reverse: bright light in the evening, a later wake time and dim mornings until your sleep has moved.
6. When you travel, dark sunglasses keep light off your eyes at the hours it would push you the wrong way. The jet lag planner gives the hours.

What about screens?

They count, but the evidence is mixed. In a 2015 study, reading on a light-emitting e-reader before bed, against a printed book, meant people took longer to fall asleep, made less melatonin, had a later clock and were less alert the next morning. In a 2016 study in Uppsala, 14 people spent 6.5 hours in bright light (about 569 lux) during the day, then read for 2 hours from 9 to 11pm on a tablet or from a printed book. Their sleep and their evening melatonin were the same either way. Is your phone what stops your melatonin? goes through both.

Do I really have to sit in the dark?