How much morning sunlight you need, and when

Morning light, soon after you wake, is the main thing that keeps your body clock on time. When Wright and colleagues took people camping, a week of natural light moved their body clocks about two hours earlier ¹.

Why morning light matters

The body clock runs a little longer than 24 hours in most people, so it needs something each day to keep it on time, and light has the biggest effect. Light in the first hour or so after a normal wake time pushes the clock earlier, and the same brightness late at night pushes it later ²³.

How bright morning light needs to be

Brightness is measured in lux. Your eyes adjust so well that a lit room feels bright, but most rooms are only a few hundred lux. Outdoors, an overcast morning still gives several thousand lux and direct sun is many times brighter than that ². The more light reaches your eyes, the more your clock shifts ⁴.

A 2022 expert panel recommended that during the day, light at eye level should reach at least 250 lux melanopic EDI, a measure that counts the kinds of light your clock responds to most ⁵. Outdoor light reaches that level even on a gray morning, but most rooms don't.

How many minutes of morning light you need

As a rule of thumb, aim for 10 to 30 minutes outside in the first hour after waking, closer to 10 on a clear, bright morning and closer to 30 when it is overcast or the winter sun is low. Don't look straight at the sun.

Nobody knows the minimum amount for healthy adults, partly because people differ so much. Sensitivity to light varies more than tenfold between people ⁵⁶. A run of days in dim rooms also makes the clock more responsive to light ⁷.

Phone lux apps measure at the phone's sensor, usually pointed at the ceiling, but what counts is the light reaching your eyes from where you are looking. They're still useful for comparing one spot with another.

If you are trying to shift earlier and you still aren't getting sleepy earlier after two weeks, move the light earlier in your morning or stay out longer, toward 30 minutes.

If you can't get outside in the morning

If you wake up before sunrise

Turn on the brightest lights you have while you get ready, then step outside once it's light, even briefly.

If you wake very early, a few hours before your usual time, keep the lights low at first. Your body temperature is lowest about two to three hours before you normally wake. Light before that point pushes the clock later. Light after it pushes it earlier ²³. So if you are up at 4 a.m. one morning and normally wake at 7 a.m., keep the lights low until about 5 a.m., when your temperature has probably started rising. If 4 a.m. is your regular wake time, bright light after waking is fine.

If it's winter or cloudy

Go out anyway and stay closer to 30 minutes. When researchers repeated the camping study in winter, people's body clocks still moved earlier outdoors ⁸.

If you can only get light through a window

Sit right next to the brightest window, facing the window, with the blinds fully open. Light drops off quickly as you move back into a room, and tinted glass cuts it further.

If you can't get outside at all

A light box can make up for some of the daylight you're missing. The boxes sold for dark winter mornings are typically rated around 10,000 lux at a set distance, often about a foot from the face, and people tend to use them for 20 to 30 minutes early in the day ⁹. Sit farther back than that and you get a lot less. Look for a box that lists lux at a stated distance and filters out UV.

Put it slightly to one side and above your line of sight while you eat or read, rather than straight in front of you.

Light boxes can cause headaches, eyestrain or jitteriness in some people. Check with a clinician first if you have bipolar disorder ⁹.