How much daylight do you need each day?

Twenty to thirty minutes outdoors within an hour or two of waking is the priority, extended to 30 to 45 on overcast days, plus more exposure through the day. Morning is by far the highest-return time.

Is sitting by a window enough for daylight exposure?

Rarely. Glass cuts transmission and light falls off sharply with distance, so a few metres inside a room puts you back at indoor levels. A bright office is 300 to 500 lux against 1,000 to 5,000 outdoors on an overcast winter day.

Is a lightbox as good as real daylight?

It works, and it is the right tool when the sun rises after you do or you work nights. But it covers a narrower field of view at a fixed distance, and outdoors adds movement and temperature change that are wake signals themselves.

Does daytime light affect how well I sleep at night?

Yes. The circadian system runs on day-night contrast, so higher daytime light exposure improves nighttime sleep. A dim day followed by a bright evening inverts the signal, which is what default indoor life produces.

How Much Daylight Do You Need?

Most people substantially overestimate how much light they get, because subjective brightness is a terrible guide. Your eyes adapt; the numbers do not.

The numbers that make this make sense

Light intensity is measured in lux, and the range across a normal day is enormous:

Direct summer sunlight: 50,000–100,000+ lux
Overcast winter day, outdoors: 1,000–5,000 lux
Brightly lit office: 300–500 lux
Ordinary home lighting in the evening: 50–200 lux
Comfortable reading light: around 100 lux

A dull, grey, unappealing winter day outside delivers roughly ten times what a well-lit office does. That single comparison does more work than any advice that follows.

This is also why "I sit by the window" rarely holds up. Glass cuts transmission, and light falls off sharply with distance from the window — a few metres in and you are back to indoor levels.

What it is actually for

Two distinct jobs, and they run on different timescales.

Circadian entrainment. Specialised retinal cells signal directly to the master clock. Morning light advances the clock and sets the timing of melatonin release roughly 14 to 16 hours later. Without a strong morning signal the clock drifts later, which is why dark mornings are so hard. (Why winter mornings are worse.)

Alertness and mood, which is more immediate. Bright light acutely improves alertness and has real antidepressant effects, which is the basis of light therapy.

How much, and when

Morning is the priority. 20 to 30 minutes of outdoor light within an hour or two of waking is the standard recommendation, and it is the highest-return time of day by a wide margin.

On an overcast day, longer. Same principle, less intensity — 30 to 45 minutes is a reasonable adjustment.

Through the day, more is better. Daytime light exposure improves sleep at night, and there is evidence that people with higher daytime light exposure have better circadian amplitude — a stronger day-night contrast, which is really what the system runs on.

No sunglasses in the morning if you are doing this deliberately. Through the middle of a bright day, wear them; the morning signal is the one you are protecting.

Windows do not substitute, but they help. Working near one is better than not.

The realistic target: get outside in the morning, and get outside again at some point during the day. Twenty minutes each is a reasonable floor. (How much and for how long, in detail.)

A 10,000 lux therapy box works, and it is the right tool when the sun rises after you do, when you work night shifts, or when you have seasonal affective disorder.

But it is narrower than daylight — a fixed distance, a fixed direction, and a fraction of the field of view that being outdoors gives you. Outdoors also comes with movement and temperature change, both of which are wake signals themselves. (The comparison in full.)

Use a box when daylight is genuinely unavailable. Do not use it as a convenience substitute.

The other half: darkness

Under-discussed and equally important. The system runs on contrast, so a bright day and a genuinely dark evening are two halves of the same intervention.

Evening light — overhead lighting, screens — suppresses melatonin and delays the clock. If your day is dim and your evening is bright, you have inverted the signal, which is roughly what modern indoor life produces by default. (The evening dimming protocol.)

Getting daylight without fixing the evening captures maybe half the available benefit.

Where this sits relative to everything else

Light timing is the highest-leverage circadian intervention there is, and it is free. It sets when you sleep well.

It does not by itself fix how you sleep. If you are getting excellent light exposure, a consistent schedule, and still waking unrefreshed, something is fragmenting the night rather than mistiming it — disrupted breathing being one of the commonest and most fixable. (What the breathing route changes overnight, with the papers cited.) (The full checklist.) (What nasal breathing changes across a night.)

The bottom line

An overcast day outdoors delivers around ten times what a bright office does, so almost nobody is getting enough by sitting near a window.

Twenty to thirty minutes outdoors in the morning, more if it is grey, and get out again during the day. Then make the evening genuinely dark — the contrast is what the system reads, and half an intervention gets you half the result.