Daylight is the main signal that keeps your body clock in time with the day. The US National Institute of General Medical Sciences puts it simply: "Light and dark have the biggest influence on circadian rhythms, but food intake, stress, physical activity, social environment, and temperature also affect them."
What matters is not only how much light you get but when you get it. Light in the morning nudges your clock earlier, so you feel sleepy earlier at night. Bright light late in the evening nudges it later. This piece is part of our complete guide to better sleep.
Why does the timing of light matter?
Your body clock has a daily low point, roughly when your body temperature is lowest, a few hours before you usually wake. A 2019 review in Frontiers in Physiology gives the example of someone who sleeps from 11pm to 7am, whose low point falls at about 4am. Which side of that low point light arrives on decides which way the clock moves:
| When the light arrives | What it does to your body clock |
|---|---|
| In the hours after the low point: early morning | Moves it earlier, so you get sleepy and wake up earlier |
| In the hours before the low point: evening and night | Moves it later, so you get sleepy and wake up later |
The review describes light "in the ~12 h prior to CBTmin", the low point, as causing a delay, and light "in the ~12 h after CBTmin" as causing an advance. For most people on a normal schedule, that makes morning daylight the light that pulls the clock earlier.
How bright does daytime light need to be?
Brighter than most indoor rooms. In 2022, a group of sleep and light researchers published the first consensus recommendations on daily light exposure in PLOS Biology. They measure light the way the body clock senses it, as melanopic lux at eye level, and suggest a minimum during the day, a maximum in the evening and near darkness for sleep.
| Time of day | Recommendation (melanopic lux at the eye) |
|---|---|
| During the day | At least 250, with daylight first where possible |
| Evening, from at least 3 hours before bed | At most 10 |
| While you sleep | As dark as possible, at most 1 |
The authors point out that indoor lighting standards are "typically at least 1 order of magnitude below the natural light environment outdoors", and that common electric lights are "typically substantially less efficient at stimulating melanopsin than natural daylight", the light-sensing pigment that talks to the body clock. In other words, a bright-looking office or kitchen is still dim by the standards of daylight outside.
The recommendations are written for healthy adults aged 18 to 55 with regular daytime schedules. The authors say children, older adults and shift workers may need different advice, and that the guidance is not intended for night shift work.
What happens when people live by natural light alone?
A 2013 study in Current Biology compared people's body clocks during normal life with electric light and after a week of summer camping with only natural light. Under modern lighting, the researchers write, "electrical lighting and the constructed environment is associated with reduced exposure to sunlight during the day" and more light after dark.
On the camping trip, "the internal circadian clock synchronizes to solar time." Biological night began around sunset and ended around sunrise, earlier than at home. Night owls shifted the most: "later chronotypes show larger circadian advances when exposed to only natural light." It was a small study in summer, but it shows how strongly ordinary daylight and darkness steer the clock.
How can you get more morning light?
- Go outside soon after waking, even briefly. Outdoor daylight is usually far brighter than indoor light.
- Walk part of your commute or take a morning walk, which also counts toward your weekly activity.
- Work or eat breakfast next to a window, facing out, if you cannot get outside.
- Open curtains and blinds as soon as you get up.
- Never look straight at the sun. The aim is to be in daylight, not to stare at it.
The ten-minute morning routine is an easy place to fit this in.
What should you do with light in the evening?
Turn it down. The consensus suggests keeping light at eye level low from at least three hours before bed and making the bedroom as dark as possible. That matters more than the colour of your screen: our article on whether blue light really ruins your sleep explains why brightness is the bigger lever, and digital sunsets covers how to wind down in practice.
Can light help with jet lag and clock changes?
Yes, because both are the same problem: your body clock is set to a different time from the clock on the wall. Timing your light exposure is one of the main ways to close the gap. The jet lag planner works out when to seek and avoid light each day of a trip, and the clock change planner does the same for daylight saving time.
When is light not the answer?
If you have long-standing trouble falling or staying asleep, feel very sleepy during the day despite enough time in bed, or notice your mood dropping in the darker months, talk to a doctor. Light timing helps a healthy body clock keep time, but it is not a treatment for sleep disorders or depression, and light therapy devices should be used with medical advice. Our article on common causes of insomnia is a good starting point.
