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MONKSEATON High School in Tyneside, UK, has seen some amazing improvements in the past year. Absenteeism is down, punctuality is up and exam results have gone through the roof. Head teacher Paul Kelley cannot attribute these successes to better teaching or stricter discipline. Instead, he simply started opening the school at 10 am instead of 9 am.
The change was designed to synchronise the school day with pupils鈥� body clocks. Teenagers are notoriously owlish, preferring to stay up into the small hours and sleep in till lunchtime. This isn鈥檛 entirely their own fault: natural delays in secretion of the sleep hormone melatonin causes their body clocks to be shifted several hours backwards (see 鈥淭eenagers: Lost in time鈥�). By aligning the school day with these biological rhythms, Monkseaton school avoids teaching teenagers when their brains are still half asleep.
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In the modern world our lives are largely dictated by time. But even in the absence of clocks, schedules and calendars, our bodies still march to the beat of internal timekeepers called circadian rhythms. Over each 24-hour period we experience cycles of physical and mental changes that are thought to prepare our brains and bodies for the tasks we鈥檙e likely to encounter at certain times of the day.
鈥淓ven in the absence of clocks, schedules and calendars, our bodies still march to the beat of internal timekeepers called circadian rhythms鈥�
The most obvious is the sleep-wake cycle, but there are many others. Circadian rhythms affect everything from how we perform on physical and mental tasks to when drugs are more likely to be effective. 鈥淲e鈥檙e not the same organism at midday and midnight,鈥� says , who researches circadian rhythms at the University of Oxford.
The main driver of circadian rhythms is a tiny patch of brain tissue called the suprachiasmatic nucleus (SCN), located just above the optic nerves. This master clock gathers information about light from the retina and relays it to the rest of the body via nerve impulses and hormones.
Among these are the sleep hormone melatonin and its opposite number orexin. The SCN also imposes its rhythms on immune function, digestion, cell division, body temperature and more. Its own pattern of activity is reset each day by light, and this influences the expression of a handful of 鈥渃lock genes鈥� whose activity follows a 24-hour cycle.
The SCN isn鈥檛 the be-all and end-all of biological timekeeping. Many of the body鈥檚 cells also contain clocks of their own which have peaks and troughs of activity throughout the day. For example, inflammation-causing immune cells called mast cells are more active in the early morning, which may be why immune disorders such as asthma are more troublesome at this time. Skin cells also show circadian rhythms, proliferating at night and producing more oil during the day, while cells in the stomach that release the hunger hormone ghrelin also seem to be controlled by a circadian clock.
These local clocks are not completely independent of the master clock. The SCN is thought to act like the conductor of an orchestra, producing a regular signal from which the rest of the musicians take their cues. 鈥淚f you shoot the conductor, the members of the orchestra will keep on playing, but they鈥檙e all playing at slightly different times so the rhythmicity falls apart,鈥� says Foster. People whose SCN stops functioning because of injury or disease lose their regular 24-hour cycle.
Not surprisingly, our physical and mental states vary widely with the time of day. For example, core body temperature is at its lowest at around 4.30 am, rises through the day and peaks at around 7 pm. Adrenalin levels also rise throughout the day.
These changes can affect how we perform on various tasks. 鈥淭here is fairly comprehensive evidence of circadian rhythms in many aspects of human performance, including athletic,鈥� says , UK. As your body temperature and adrenalin levels rise during the afternoon, physical performance tends to improve. Meanwhile the ability to carry out complicated mental tasks like decision-making is negatively affected the longer you have been awake.
Not everyone follows the same pattern. Some people are larks, preferring to rise early and retire early, while owls find it difficult to function in the morning but thrive late at night. These 鈥渃hronotypes鈥� are largely determined by genes. Most of us fall somewhere in the middle.
At the extreme end of the spectrum are people with a rare but somewhat treatable disorder called familial advanced sleep-phase syndrome (FASPS), who wake naturally in the early hours of the morning and fall asleep in the early evening. We now know that FASPS is caused by a single mutation in a gene called PER2, one of a handful of clock genes responsible for setting the SCN.
Clocks can also be nudged forwards by exposure to bright light in the early morning, though preliminary evidence suggests that some people鈥檚 clocks are more resistant to resetting than others, says in Switzerland. This might explain why some people are more susceptible to jetlag and find it harder to adapt to shift work than other people.
Age can also cause profound shifts in your body clock. Older people tend to sleep less and wake earlier. Brown鈥檚 lab recently discovered a factor in the blood of elderly people that can shift the circadian rhythms of skin cells towards the lark end of the spectrum ().
This discovery suggests it might be possible to develop drugs that turn owls into larks and vice versa. 鈥淭hat could be useful not only for older individuals, but for shift workers and people with sleep syndromes,鈥� says Brown. Though don鈥檛 hold out any hope of sleepy teens ever being bright-eyed and bushy-tailed at 9 am.
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