Tick-tock goes the body clock

You can sleep when you’re dead, they say. In the meantime, though, circadian rhythms are best not tampered with.

Getting by with little sleep and therefore adding more productive hours to your day is often seen as a badge of honour. It is said Margaret Thatcher and Napoleon Bonaparte slept only four hours a night. For many of us who can’t seem to shake that groggy feeling before mid-morning, that seems wildly unfair.

There is no set sleep prescription for humans. The amount of sleep we need, as well as the timing of that sleep, changes as we age. Newborns need up to around 20 hours a day, teenagers nine and senior citizens struggle to get all their sleep in one block.

Even among adults, there is considerable variation, according to Till Roenneberg, professor of chronobiology and medical psychology at Munich’s Ludwig Maximilian University. Looking at the sleep patterns of more than 150,000 people he found that 11% of adults slept for six hours or less, around a quarter slept eight hours or more, and the majority fell somewhere in between.

There’s a simple way to check whether you’re getting enough sleep: you should wake up naturally, feeling refreshed.

Sleep is regulated by two factors: our circadian rhythm, which is tied mainly to light/dark cycles, and how long we have been awake, also known as sleep pressure. The body clock is set on average 10 minutes longer than 24 hours, although in about a quarter of people it is slightly less. Much of this variation is genetic, but it is also influenced by environmental factors such as how much artificial light we are exposed to. Light affects our circadian rhythm through the “master clock” in an area of the brain called the suprachiasmatic nucleus (SCN). Light hitting the retina tells the SCN to delay production of the night-time hormone melatonin, interfering with sleep.

Getting by on little sleep could have unseen health effects. “Four hours of sleep may be sufficient for you to sit in the House of Commons,” says Derk-Jan Dijk at the University of Surrey, “but it may have a negative effect on, for example, your risk of cardiovascular disease.” The health effects of sleep deprivation are made worse by what Roenneberg and his colleagues have come to term “social jet-lag” – the discrepancy between when our bodies are programmed to sleep and when our social programme wants us to. This is particularly problematic for people with extreme chronotypes, says professor Anna Wirz-Justice, who studies chronobiology at the Psychiatric Hospital, University of Basel. “These are the owls and the larks who are a couple of hours out of sync, and have real problems adjusting to school and work”.

Wirz-Justice and her colleagues on a European-wide project called Euclock have been developing technologies to better monitor body clocks. For instance, they developed a way to test biological clocks using skin cells, as well as light meters at eye level hooked onto glasses, which measure how much and what wavelength of light a person is getting. That enables scientists to see how an individual’s behaviour and light exposure affect their circadian phase, sleep quality and mood.

To make the most of the new understanding of circadian rhythms, we need to see a shift in attitude towards sleep, Wirz-Justice says. She has championed a change in school start times to better reflect the fact that teenagers are built to sleep later in the day. That idea has been met with predictable resistance from the teachers and parents who would also be affected. Even so, the idea that surviving on little sleep is a badge of honour is starting to go out of fashion, she says. It’s time to put the “sleep when you’re dead” mentality firmly to bed.

Derk-Jan Dijk (Sleep Research Centre at the University of Surrey), Till Roenneberg (Ludwig Maximilian University), Anna Wirz-Justice (Psychiatric Hospital of the University of Basel)