Daylight and sleep routines: what is worth understanding

The body clock, in a few words

The body has an internal clock, in the brain, that organizes sleep and wakefulness and many other rhythms over the course of the day. The light that enters the eyes reaches it through cells in the retina that are different from the ones we use to see, and it's the main signal that keeps the clock in sync with the 24-hour cycle.¹

That clock doesn't run on exactly 24 hours. In a study that measured it in 157 adults, who stayed in a laboratory for two to six weeks under a protocol designed to measure it without interference, its cycle lasted 24 hours and 9 minutes on average, and almost four out of five people had one somewhat longer than 24 hours.² That's why it needs a daily reset: according to the authors, in people with a longer cycle the reset means moving it earlier, and morning light does that.²

To know where the clock stands, researchers usually measure the melatonin the body itself produces: it starts to rise when the biological night begins and helps sleep to start.⁸ Melatonin supplements are a different matter, which this article doesn't cover.

What matters most is the timing. In a classic laboratory experiment, 21 healthy adults received a single exposure to very bright light (5,000 to 10,000 lux) for almost seven hours, each at a different time. The light delayed the clock when it came before the lowest point of body temperature, which usually falls in the early hours of the morning, about two hours before the usual wake-up time (our rough calculation from the study's data), and moved it earlier when it came after: a delay of up to 3.6 hours and an advance of up to 2. It's an extreme exposure: it shows the direction of the effect, not how much the clock moves in normal life.³ An international group of experts that drafted public health messages on light in 2025 sums it up this way: morning light can move the clock earlier, and evening light can delay it.¹

What morning and daytime light do

Outside the laboratory, much less has been measured: the same group of experts points out that most studies on the effects of light have been done in the laboratory, although they agreed that more light during the day can improve the quality of the following night's sleep.¹ The real-life evidence is mainly of two kinds.

• Camping without electric light. A team at the University of Colorado compared 8 adults during a week of normal life in summer and a week of camping with only natural light and campfires. While camping, they received more than four times as much light during their waking hours, about three times as much in the first two hours after waking up and less between sunset and bedtime. Their clock moved about two hours earlier and they went to bed and got up around 1.2 hours earlier, but they slept the same amount.⁴ The same team found that, in winter and with 5 people, melatonin started to rise about 2.6 hours earlier; and after a summer weekend of camping, 1.4 hours earlier in the 9 who camped, while it was delayed by almost an hour in the 5 who stayed at home.⁵
• A large observational study. In the UK Biobank, with more than 400,000 British adults aged 37 to 73, each additional daily hour outdoors, as they reported it, was associated with finding it easier to get up, feeling tired less often, having trouble falling asleep or waking up in the middle of the night somewhat less often, and describing themselves as more of a “morning person.”⁶ The relationship went both ways: those who had more sleep difficulties at the start later spent less time outdoors.⁶

Why evening light matters

In the evening the opposite happens: light holds back the rise of melatonin and can delay the clock,¹ and it doesn't need to be bright. In an analysis of two laboratory studies with 116 healthy adults aged 18 to 30, spending the eight hours before bed in the light of an ordinary room (less than 200 lux) instead of in dim light (less than 3 lux) made melatonin start to rise later in 99% of participants and shortened the time during which the body produced it by about 90 minutes.⁷

In another experiment, with 55 adults aged 18 to 30, about 25 lux on average was enough to cut melatonin by half, an intensity comparable to or lower than the usual lighting of a home in the evening, according to the authors. But the differences between people were huge: for the most sensitive person, about 6 lux was enough for that same effect, and the least sensitive needed about 350.⁸

Melatonin rising later isn't the same as sleeping worse, but it points in that direction: the 2025 experts agreed that more light in the evening can lengthen the time it takes to fall asleep.¹ Screens are one of those sources of light, although with them what you do also counts; we cover that separately in screens at night.

It's often said that getting a lot of light during the day protects you from evening light. The 2022 consensus considers it a possible added benefit, with some studies in favor, but acknowledges that it isn't well understood how it works or how much light it takes;⁹ in the experiment with the 55 young adults, sensitivity wasn't related to prior light exposure,⁸ and in 2025 the group of experts dropped that statement for lack of sufficient evidence.¹

What the experts propose

In 2022, a group of experts published recommendations on indoor light, intended for healthy adults aged 18 to 55 on daytime schedules.⁹ They express them in a measure that weights light by how much it stimulates the retinal cells that inform the clock (the “melanopic equivalent daylight illuminance”), taken at eye level:

• During the day: at least 250 lux in that measure, preferably natural light.⁹
• In the evening, starting at least three hours before bed: no more than 10 lux.⁹
• For sleep: as dark as possible, below 1 lux; and if you have to get up and need to see, no more than 10.⁹

This isn't the figure an ordinary light meter gives: for the same lux, natural light stimulates those cells considerably more than the most common sources of electric light.⁹ According to the same consensus, indoor lighting standards usually set levels at least ten times lower than natural outdoor light, and many warm-white household LED bulbs (2,700 to 3,000 K) make it possible to meet the evening target with lighting of about 30 lux.⁹

What you can try in your daily life

What follows are our own inferences from the sources, not guidelines tested in trials:

• Light early in the morning. Going outside in the first hours after you get up, even if only on your way somewhere on foot, is the most direct way to get light at the time when it moves the clock earlier.³⁴ None of our sources specifies how many minutes. In winter, if you leave home before sunrise, that light may come later: on the way, in the middle of the morning or during a break outdoors.
• More light during the day. There's much less light indoors than outdoors;⁹ working near a window or moving something you already do indoors outside adds light without adding tasks.
• Dim light in the last hours. Lowering the intensity, choosing, instead of the ceiling light, lamps that don't shine directly into your eyes, and using warm-white bulbs brings your home closer to what the consensus proposes.⁹
• Darkness for sleep. Blinds or curtains that block the light from the street and, if you need a night light to find your way, a dim one.⁹

The Sociedad Española de Sueño (Spanish Sleep Society) proposes something similar depending on chronotype: for strong evening types, bright light first thing in the morning; for strong morning types who want to shift their schedule later, avoiding the sun first thing in the morning and seeking more bright light at the beginning of the evening.¹⁰

If you want to check whether any of these changes works for you, write down your schedules and how you sleep for a few weeks; in how to keep a sleep diary we offer a simple template. If what changes is your schedule, because of the clock change, a trip or an early start you can prepare for a few days in advance, light also helps you adjust: we cover this in how to prepare your sleep when your schedule changes. And if you get up much later on your days off, the light of those mornings is one of the ideas in sleeping differently on weekends.

Limits and when to consult

Almost everything that is known with precision comes from laboratories and from healthy young adults,¹⁹ and the real-life evidence we've cited consists of very small or observational studies.⁴⁵⁶ In addition, people respond to light very differently, and with age less light reaches the retina.¹⁸ That's why these ideas are something to try, not a guaranteed result.

This article is about the light of daily life. Bright-light lamps used as a treatment for some health problems, with a protocol prescribed by a professional, are outside its scope.¹ If you work shifts, the consensus recommendations aren't designed for you;⁹ raise it with your company's occupational health or risk prevention service.

If you've had trouble falling asleep or waking up for weeks or months, if your sleep schedule has shifted a lot and you can't move it back, or if this affects your daily life, consult a health professional; in the general article on sleep we explain when it's a good idea to do so.