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The Rhythms of Life

13 Oct

Linda Geddes – New Scientist

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 Kelly cannot attribute these successes to better teaching or stricter discipline, instead, he simply started opening the school at 10am instead of 9 am.

The change was designed to synchronise the school day with pupils’ body clocks. Teenagers are notoriously owlish, preferring to stay up into the small hours and sleep in till lunchtime. This isn’t entirely their own fault: natural delays in secretion of sleep hormone melatonin causes their body clocks to be shifted several hours backwards. By aligning the school day with this biological rhythms. Monkseaton school avoids teaching teenagers when their brains are still half asleep.

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 the 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’re likely to encounter at certain times of the day.

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. “We’re not the same organism at midday and midnight,” says Russell foster, who researches circadian rhythms at the University of Oxford.

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The main driver of circadian rhythms is a tiny patch of brain tissue called suprachiasmatic nucleus (SCN), located just above the optic nerves. This master clock gathers information about the 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 the 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 “clock genes” whose activity follows a 24-hour cycle.

The SCN isn’t the be-all and end-all of biological timekeeping. Many of the body’s cells also contain clocks on their own which have peaks troughs of activity throughout the say. 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 nigh 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. “If you shoot the conductor, the members of the orchestra will keep on playing, but they’re all playing at slightly different times so the rhythmicity falls apart,” 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 the day. For example, core body temperature is at its lowest at around 4.30 am, rises through the day and peaks at around 7pm. Adrenalin levels also rise throughout the day.

These changes can affect how we perform on various tasks. “There is fairly comprehensive evidence of circadian rhythms in many aspects of human performance, including athletic,” says Jim Waterhouse of Liverpool John Moores University, 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 “chronotypes” 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 clock genes responsible for setting the SCN.

Clocks can also be nudged forwards by exposure to bright light in the early morning, through preliminary evidence suggests that some people’s clocks are more resistant to resetting than others, says Steven Brown of the University of Zurich 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’s 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 (Proceedings of the National Academies of Science, Vol 108, P 7218).

This discovery suggests it might be possible to develop drugs that turn owls into larks and vice versa. “That could be useful not only for older individuals, but for shift workers and people with sleep syndromes,” says Brown. Though don’t hold out any hope of sleepy teens ever being bright eyes and bushy tailed at 9 am.

Linda Geddes – New Scientist (8th October 2011)

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