Bright light in the morning moves your body clock earlier, so you get sleepy at night and wake up on time. Outside, daylight is many times brighter than a lit room, even under cloud. That makes the hours after first light the ones to get outside for.
Why morning light?
Your body runs on a clock of roughly 24 hours, and light is the main signal that keeps it in time. The NIH's National Institute of General Medical Sciences names light and dark as the biggest influence on these circadian rhythms.
When the light arrives decides which way the clock moves. Training material from the CDC's NIOSH puts it simply: bright morning light moves the clock earlier, so you get sleepy earlier in the evening and wake earlier. Bright evening light moves it later. It estimates the morning shift at up to about an hour a day, and the evening shift at up to about two.
Is light through a window enough?
Usually not. A 2021 study in Proceedings of the Royal Society B that measured people's light through the day put bright indoor lighting at around 1,000 lux, while daylight outdoors, even in shade, regularly reached ten to a hundred times that. The people it followed rarely got more than indoor levels, because they spent so little time outside.
The light boxes used to treat winter depression give 10,000 lux, which the National Institute of Mental Health describes as about 20 times brighter than ordinary indoor light. A bright day outside gets there on its own. A window lets in part of it, and less the further from the glass you sit.
How long should you spend outside?
There's no official number for healthy adults. The closest guide is light therapy for seasonal affective disorder, which the NIMH describes as 30 to 45 minutes in front of a 10,000 lux box, usually first thing in the morning, from fall to spring.
Outdoors, cloud matters more than anything else: a bright morning gives far more light per minute than a grey one. The DaySight app suggests minutes for each morning from the cloud forecast for your place, so a grey day asks for longer.
When does morning light start?
At first light, when the sun is 6° below the horizon and the sky is bright enough to see by. It comes about half an hour before sunrise in most of the US. In New York, first light is at 6:46 AM on the shortest day and 4:52 AM on the longest. The clock changes move it by an hour twice a year.
New York's first light runs from 4:52 AM to 7:04 AM across 2027
Clock times of first light and sunrise for every day of 2027. The gaps in March and November are the clock changes.
First lightSunriseLight before the sun is up7 AM
Source: DaySight, calculated from the sun's position for each city.
Calculated by DaySight for each city. Anchorage's sky never gets darker than first light in late June, so there's no first light to count.
Winter is when it's hardest to catch. See morning sunlight in winter for how many mornings it's still dark at 7 AM where you live, and what to do about it.
What about evening light?
Daylight late in the day still counts toward the light you get, but bright light in the evening pushes the clock later, the opposite of the morning. Keeping lamps and screens dim after dark means less of that pull.
DaySight is general information, not medical advice. If you have trouble sleeping, work shifts or feel low through the winter, talk to a doctor. The sources behind this page are on About DaySight.