Light in the evening

If your evenings run under the overhead lights right up to bedtime, with a phone or tablet in your hand for some of it, the room probably doesn't feel especially bright. Your eyes adjust, so it seems normal. Your body clock reads the same light differently, and for some people an ordinary evening at home is enough to push sleep later.

Why the clock still sees room light

Your body clock is set mainly by light reaching your eyes. Alongside the cells you see with, a small set of cells in the retina (the light-sensitive layer at the back of the eye) contain melanopsin, a light-sensitive pigment, and pass a signal of day or night to the brain's master clock (Brown et al., 2022). These cells keep working in the evening.

When enough light reaches them in the hours before bed, the clock holds back melatonin, the hormone that signals night and helps open the way to sleep, so its evening rise starts later. The clock itself also shifts later, which moves the time you're likely to feel sleepy the next night as well.

What the studies found

Gooley and colleagues (2011) looked at 116 healthy adults aged 18 to 30 who lived in a research unit for at least five days. They compared ordinary room light (under 200 lux, a measure of how much light falls on a surface) with dim light (under 3 lux) in the eight hours before bed. Room light delayed the start of melatonin in 99% of them and shortened the body's own melatonin night by about 90 minutes.

It doesn't take much light. Phillips and colleagues (2019) exposed 55 young adults to a range of evening light levels for five hours. On average, about 25 lux was enough to halve melatonin, and 30 lux pushed its apparent start about 77 minutes later. People varied enormously: the most sensitive person's melatonin was halved by 6 lux, the least sensitive needed 350. Two people sitting in the same room can respond to it very differently, which is one reason we rely on your diary rather than a fixed rule.

Screens add to this, though they often aren't the brightest thing in the room. In a laboratory study, 12 healthy young adults read for about four hours before bed on five evenings, once from a light-emitting tablet and once from a printed book (Chang et al., 2015). With the tablet they took about ten minutes longer to fall asleep, made less melatonin, their body clock ran later and they were less alert the next morning. That tablet was set to full brightness in an otherwise dim room, so it shows what a screen can do more than what yours does.

We're firm on the physiology: evening light holds back melatonin and moves the clock later, and the effect grows with brightness. How much a change at home improves sleep for any one person is less certain, and that's what your diary is there to show.

Lamps on low

An international panel of experts, drawing on this body of research, recommends keeping evening light dim from at least three hours before bed and making the bedroom as dark as possible while you sleep (Brown et al., 2022). The same group notes that many ordinary warm white LED bulbs, at the levels people typically use at home, already meet their evening target. You don't need special bulbs or glasses.

So your plan works from about three hours before your usual bedtime. Overhead lights are usually the brightest thing in the room, so that's where we start: switch them off and use lamps on low, ideally below eye level. Screens are usually the harder habit to change, so we come to them once the lamps are settled. Each step is small, and your diary will show whether your sleep starts to come earlier or settle more quickly.