How does light affect my sleep?

Perhaps night mode went on months ago, and a rule about no phone in bed lasted a week before it lapsed. There may have been orange glasses too, and you still found yourself awake past midnight, half-suspecting you’ve been doing it wrong. When the house is asleep, the phone is often the one thing keeping you company, so advice to put it away feels like losing something. What’s harder to notice is the other end of the day, and how little daylight there was in it.

A phone glanced at in the night, or read with the brightness turned down, matters less than you may fear. The best-known screen study asked much more of the people in it. Twelve young adults read on a light-emitting e-reader at full brightness for about four hours each evening¹1 Chang et al., 2015, in a room kept very dim. Afterwards they took about ten minutes longer to fall asleep than after a printed book, and their total sleep stayed the same. That is a real effect, and it came from hours at full brightness, a long way from a few minutes on a dimmed phone. The study didn’t test anything dimmer, warmer or shorter¹, so it can't tell us how much those few minutes matter.

If the phone takes anything from your night, the studies suggest it is mostly time. In a survey of over 45,000 Norwegian students²2 Hjetland et al., 2025, each extra hour on a screen after going to bed went with about twenty-four minutes less sleep. It made little difference whether people were scrolling, watching or messaging. The researchers read this as an hour on the screen being mostly an hour not spent asleep, with the light playing a smaller part. So what helps is within easy reach: once you start to feel drowsy, you can lay the phone on the nightstand and let that time go to sleep.

The link may run the other way as well. In a model that hasn’t been put to the test yet, researchers propose that when sleep won’t come, people reach for the phone for company, or to take their mind off thoughts that won't stop³3 Bauducco et al., 2024. Some people use their screens to help them drop off², and the phone may be a response to lying awake as well as a cause of it. If you pick up the phone when sleep won’t come, that doesn’t by itself mean the phone is what woke you.

You may have noticed sleepiness arriving a little later each week, until midnight starts to feel early. Light has a good deal to do with that. It reaches the clock through your eyes, by way of a small set of cells in the retina that take no part in seeing pictures⁷⁸7 Brown et al., 20228 Do & Yau, 2010. They sense light themselves, gather signals from the cells you see with, and pass word of the day to the clock. Their pigment, melanopsin, responds most to blue-green light⁷, which is where all the talk of blue light comes from.

What that light does depends on the hour it arrives⁹¹⁰9 Khalsa et al., 200310 Burns et al., 2021. Light in the evening and the first part of the night nudges the clock later, so sleepiness comes later the next night. Light after the night’s low point, in the early morning, nudges it earlier. Most people’s clocks run a shade longer than a day, so without morning light to draw them back they tend to drift later, night by night¹⁰.

A small study shows what happened when a group of campers got their dark evenings back. One July, eight adults spent a week in the Colorado Rockies with only sunlight and campfires¹¹11 Wright et al., 2013, without flashlights or phones, after a week of their usual lives. Out there they took in more than four times as much light while awake, and less after sunset. By the end, their melatoninmelatonin A hormone your brain releases as darkness falls, telling the rest of your body that night has come. It's also sold as a tablet. began near sunset and stopped just after sunrise, before they woke. Their clocks had moved about two hours earlier, furthest for those whose clocks had run latest. It was a small group in midsummer¹¹, and being more active and more sociable outdoors may have played a part. Still, why would the latest clocks move most? They were the ones furthest from the sun’s own day, so bright mornings and dark evenings had the most to draw back.

A working day can pass with hardly any daylight in it: a commute in the half-dark, a desk far from the window, lunch at the screen, and your body left with very little sign that it was ever morning. Indoors, lighting standards keep rooms at least ten times dimmer than outdoors⁷7 Brown et al., 2022, and office light does less for the clock than daylight of the same brightness. Many people spend their days in less light than researchers recommend for healthy adults on daytime schedules⁷, especially if their sleep already runs late. If that’s you, your days are where this matters most, since morning light is what draws sleepiness earlier.

Daylight seems to steady the evening as well. In one small trial, fourteen people spent a day in bright light and then read for two hours before bed¹²12 Rångtell et al., 2016, once on a tablet and once from a paper book. Their melatonin and their sleep came out the same either way. Other studies have found the same buffering⁷, though how much daylight it takes, and at what time of day, hasn’t been worked out.

Two studies of everyday life point the same way, though neither can show cause and effect. In the Netherlands, twenty people wore light sensors and had their sleep recorded at home¹³13 Wams et al., 2017. Those whose first real light of the day came later had later clocks and woke more often in the night. In the UK, researchers looked at over 400,000 adults¹⁰10 Burns et al., 2021, and each extra hour spent outdoors in daylight went with finding it easier to get up, feeling less tired and slightly fewer insomnia symptoms.

Evenings at home often run under the overhead light, through dinner and the dishes and the last hour on the sofa, and the clock notices. In one laboratory study of fifty-five young adults¹⁴14 Phillips et al., 2019, five hours of evening light at the brightness of an ordinary home pushed the clock’s evening signal later. The brighter the room, the bigger the delay, and people differed more than tenfold in how sensitive they were. Those levels are about the same light as a phone held in a dark room gives at the eye⁴⁷4 Gringras et al., 20157 Brown et al., 2022, as the figure above shows, but the delays came from five hours of that light, all evening. So the room’s light across the evening is likely to count for more than a few minutes on a dimmed phone. Like other studies of its kind, this one measured melatonin, which marks the clock’s timing, rather than sleep itself.

The light researchers advise bright light through the day⁷, ideally daylight, then dim, warm light in the last few hours before bed and a bedroom as dark as you can make it. Your living room may already be close to the evening part: one lit by warm-white bulbs, commonly around thirty lux at the eye, is already about as dim as the researchers advise for the evening⁷. Kitchens and other rooms can be brighter.

A hall light under the door, a night light or a quick light for the bathroom hasn’t been shown to matter, and lights this small haven’t really been studied. The one small laboratory study of light during sleep looked at something much brighter. Twenty young adults slept a night either in dim light or with the overhead light on at about 100 lux¹⁸18 Mason et al., 2022, as bright as a fully lit room and far brighter than a night light. With the light on, their hearts beat faster through the night and their bodies handled sugar less well the next morning. Their melatonin was unchanged, and their sleep was no more broken. Closed eyelids let through only about five to nine percent of light¹⁸, so the study says little about a hall light or a night light. What it does give is one modest reason to sleep with the main light off.

If your bedroom is a little brighter than you’d like, anything that darkens it will do, blackout blinds among them, with a low light where you need one⁷7 Brown et al., 2022. If you get up in the night, seeing your way comes first: a dim, warm light aimed low and away from your eyes does the job, and older people may need a little more light than that to reach the bathroom safely⁷.

If you work nights, the light tends to fall at the wrong end of your day, and you may know it well: the drive home with the morning sun full in your face, arriving at the pillow wide awake, and a bedroom that doesn’t quite go dark at noon. Even over several nights in a row, the body clock barely shifts toward a night schedule¹⁹19 Boivin et al., 2022, so it stays out of step with both the work and the sleep that follows.

The clearest picture comes from a laboratory trial in which sixty-seven young adults worked five simulated night shifts. Some worked under bursts of bright light, twenty minutes on and forty off, and others under standard room light. All of them wore sunglasses on the way home and slept from 8:30am in darkened bedrooms. With bright light at work, almost everyone whose clock needed to move adapted fully to the night schedule²⁰20 Crowley et al., 2003. A smaller follow-up suggests that even a partial shift brought mood, fatigue and performance on night shifts close to daytime levels²¹21 Smith et al., 2009.

How much this helps people working real shifts, with real lives around them, is less certain. A Cochrane review found the trials of bright light for shift workers mostly low or very low quality²²22 Slanger et al., 2016 and couldn’t say whether it improves their sleep, and the light researchers’ consensus treats its advice for night workers as still unsettled⁷7 Brown et al., 2022. Rosters, naps and caffeine are part of the same picture, and How do I sleep well on night shifts? takes those up.

Since the laboratory results are clear, it may well be worth trying for yourself. During the shift, you could work under as much bright light as your workplace allows, which for most people means brighter workplace lighting rather than special lamps. On the way home, ordinary sunglasses can keep the morning out, though nothing darker than you’d be safe to drive in. At home, blackout blinds or anything else that darkens the room can keep it dark right through the day.

Light shapes when sleepiness arrives, and if insomniainsomnia Trouble falling asleep, staying asleep or waking too early, that keeps happening and affects how you feel or get on during the day. has gone on for months, it may help to look beyond the lamps to CBT for insomniaCBT for insomnia Cognitive behavioral therapy for insomnia: a short course of practical, talking-based techniques that help retrain sleep. It’s the first treatment doctors recommend for long-term insomnia., the first treatment doctors recommend for it²³23 Qaseem et al., 2016, which What is CBT for insomnia, and what would it ask of me? explains.

For most nights, though, the thing worth trying tomorrow is the morning. Daylight early in the day is what draws sleepiness earlier, and more of it may do more for your nights than anything you change about the screen.