How daytime light affects your sleep at night

People who get more daylight tend to fall asleep sooner and rate their sleep better. Office workers who sit far from a window often get very little of it.

How bright is it indoors compared with outside?

Most rooms get a small fraction of the light you would get outside, even on a gray day. The morning sunlight guide gives the numbers. Desks away from windows often fall short of the daytime light level a 2022 expert panel recommended ¹.

Does daytime light help you sleep better?

Researchers in the US had 109 office workers in five government buildings wear a device that measured the light reaching their eyes, for a week in winter and again in summer. Those who got more light in the morning fell asleep faster, especially in winter, and rated their sleep better. Those who got more light across the whole day also reported better sleep and lower depression scores ².

In another US study of 49 office workers, the 22 who worked near windows reported better sleep than the 27 who worked in windowless offices or far from any window. Of the 49, 21 also wore wrist monitors, and the ones near windows got more light during the work week and slept longer at night ³.

Both office studies only compared groups of people, so they can't show the light itself made the difference. A small Dutch study measured sleep more directly, though it was also observational. Researchers recorded the sleep of 20 healthy people at home with brain-wave equipment, while wrist devices tracked their light. People whose first light over 10 lux came later in the day woke up more often during the night. Earlier first light and brighter peak light were followed by more deep sleep ⁴.

Does bright light during the day make you less sensitive to light at night?

Your body clock adjusts how sensitive it is to light based on how much light you've had over the past few days. If you've had several dim days, evening light affects it more.

In one experiment, 12 people spent a week getting over four hours of bright light a day, outdoors and from light boxes, and another week wearing dark goggles whenever they went outside. Then they sat under 500 lux of light at night. Their melatonin was suppressed more after the dim week than after the bright week, although some people barely changed ⁵.

In a Harvard lab study, when people had spent the days before in very dim light, the same bright light later shifted their body clock 60% to 70% more than it did after days in ordinary room light ⁶. A study in northern Japan found the same evening light suppressed melatonin more in winter, when people got much less daylight, than in summer ⁷.

You still need to keep your evenings dim, and how to dim your lights in the evening covers that.

How to get more daylight on a workday

Going outside soon after you wake has the biggest effect on your body clock, and how much morning sunlight you need covers how long to stay out.

Outside is brighter than your desk even in the shade, so look for chances to get out during the day, like walking part of the way to work or eating lunch outside.

Sit near a window with the blinds open, facing the window or side-on to it. At home, that may mean moving your laptop to the brightest room. Home offices are often in a spare room or corner that doesn't get much daylight.

If you work in a windowless space, a bright desk lamp or light box at the start of the day can make up some of the difference. Night shifts are different, and a doctor or occupational health service can help you work out when to get light and when to avoid it.

In winter your commute may be in the dark both ways, so lunchtime may be your best chance to get outside.