The circadian rhythm: your internal clock

Sleep is not only a matter of how tired you are. It is also a matter of what time your body thinks it is — and those two things come from different systems. Sleep pressure builds the longer you stay awake, but a separate internal clock decides when that pressure is allowed to win. This is why you can be exhausted at four in the afternoon and wide awake at midnight, and why crossing time zones feels the way it does.

The clock is a physical structure. The suprachiasmatic nucleus, a small cluster of cells in the hypothalamus, runs a self-sustaining cycle of roughly 24 hours — close to a day, but not exactly, which is the origin of the word circadian. Because it is only approximate, it has to be reset continually against the outside world, and the signal it uses is overwhelmingly light. Specialised light-sensitive cells in the retina, separate from the ones that produce vision, report ambient brightness directly to the clock. This is why blind people with no light perception at all often experience a drifting sleep-wake cycle: the clock still runs, but nothing corrects it.

Light does not just switch the clock on and off; the timing of the exposure determines which way the clock moves. Bright light in the morning tends to pull the rhythm earlier, while bright light late in the evening tends to push it later. That single fact underlies most of what is worth knowing practically about light and sleep, including the evening-screens question covered separately in this section.

The molecular machinery behind the clock is well enough understood to have won a Nobel Prize. In 2017 the Nobel Prize in Physiology or Medicine went to Jeffrey Hall, Michael Rosbash and Michael Young for their work on the molecular mechanisms controlling circadian rhythm — a feedback loop of genes and proteins that builds up and breaks down over roughly a day, first worked out in fruit flies. It is one of the more satisfying stories in modern biology: an abstract behavioural rhythm turned out to have a concrete chemical oscillator underneath it, present in cells throughout the body and not only in the brain.

Chronotype — being a lark or an owl — is real, partly genetic, and not a character trait. People differ substantially in when their internal clock places their sleep window, and that difference has a heritable component rather than being purely a matter of discipline. It also shifts predictably across a lifetime. Children tend to be relatively early, adolescence pushes the clock markedly later, and it drifts earlier again through adulthood and into old age. The stereotype of the teenager who cannot get up in the morning describes a biological shift at least as much as a motivational one.

That mismatch has a name. Till Roenneberg is closely associated with the concept of social jetlag: the gap between the time your body clock wants and the time your obligations demand, measured roughly as the difference between when you sleep on working days and on free days. Someone who wakes at seven for work but naturally sleeps until ten at the weekend is living, in effect, in a permanently shifted time zone. Larger gaps have been associated with worse health and wellbeing measures in population research, though the usual caution applies about drawing causal conclusions from associations.

The practical implications are modest but genuine, and they mostly concern regularity and light rather than willpower. A consistent wake time anchors the clock more effectively than a consistent bedtime, because waking is when light exposure begins. Morning daylight is a stronger and cheaper signal than most people assume, and the difference between outdoor light and indoor lighting is far larger than it looks to the eye. And a fully nocturnal schedule is difficult to sustain not because night workers lack discipline, but because they are asking a light-driven system to run against the light.

None of this changes what your dreams are about. It changes when you have them, how much REM you get, and how likely you are to wake in the middle of one — which, as the article on sleep cycles explains, is most of what determines whether you remember a dream at all.

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What this is based on

• The 2017 Nobel Prize in Physiology or Medicine awarded to Hall, Rosbash and Young
• Till Roenneberg's work on chronotype and social jetlag
• Chronobiology research on light entrainment of the suprachiasmatic nucleus