Your body clock and light

Sleep keeps a time of its own. If you've stayed up well past your usual hour you may have felt a second wind arrive, and if you went to bed three hours early you'd probably lie there wide awake. That sense of a right time for sleep comes from the body clock, and the main thing that sets the clock is light.

A clock that runs slightly long

The body clock is a small cluster of nerve cells deep in the brain called the suprachiasmatic nucleus (the body's master clock). It keeps a rhythm of roughly a day and uses it to time when you feel alert, when you start to feel sleepy, and when melatonin (the hormone that signals night) is released in the evening.

Roughly a day isn't exactly a day. Czeisler and colleagues (1999) studied healthy young and older adults living in carefully controlled lighting, so that light couldn't reset their clocks, and found the clock ran on an average cycle of 24.18 hours in both age groups. Left to itself the clock drifts later, so it has to be brought back into line every day, and light reaching the eyes is the main signal it uses to do that.

The route from eye to clock

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 their signal directly to the master clock and, through it, to the systems that govern melatonin, alertness and body temperature (Brown et al., 2022). They respond most to bright light rich in blue wavelengths. Daylight carries far more of that than indoor lighting does, even on a grey morning, so time outside is much the stronger signal, and the clock in turn reads a morning spent indoors as a dim one.

What the signal does depends on when it arrives. Khalsa and colleagues (2003) gave 21 healthy adults a single long session of bright light at different points in their body-clock day, in a tightly controlled laboratory, and measured how far the timing of their melatonin rhythm moved. Light in the biological evening and the first part of the night moved the clock later. Light after the low point of the night, which normally falls in the early hours before you wake, moved it earlier, and light right at that low point moved it very little. In everyday terms, daylight soon after waking pulls the clock earlier and bright light late in the evening pushes it later.

A week of natural light

Wright and colleagues (2013) looked at what modern lighting does to this. Eight adults spent a summer week camping in Colorado, living only by natural light, and their body clocks were measured against their own normal week at home. Away from electric light they got far more daylight and much less light after sunset. Their clocks moved about two hours earlier, so that the internal night began around sunset and ended just after sunrise, and those with the latest clocks moved the most.

It's a small study with no separate control group. Camping changed the evening as well as the morning, and the authors note that activity and social routine may have played a part, so it tells us the direction of the effect more clearly than how much a morning walk alone would do.

How firm this is

We're confident about the mechanism, which has been mapped in controlled laboratory work over decades. An international panel of experts, drawing on that work rather than testing it in a single trial, recommends bright light during the day and dim light in the evening for healthy adults (Brown et al., 2022). We're less sure how much one change at home does for any one person, which is why your plan tests it against your own diary.

Where your plan starts

Your waking time is the anchor. When you get up largely decides when your eyes first meet daylight, so a steady waking time gives the clock the same morning signal every day. Pairing it with daylight soon after waking, outside if you can, strengthens that signal at no cost. The evening is the other half: keeping lights low in the last few hours before bed stops the clock being pushed later again (there's more on that in Light in the evening).

We make these changes in small steps, and your diary will show over the following weeks whether your sleep times start to come forward. If you find you can't fall asleep until the early hours and struggle to wake for work however steady you keep things, or you're very sleepy in the day despite enough time in bed, talk to your GP. A clock that runs persistently very late can be a sleep condition in its own right and is worth having checked.