Leave your body clock alone, says Nick Walker
Since the beginning of time, our bodies have been regulated by rhythms - the sun, the moon and the seasons. But as anyone who has ever suffered from jet lag or worked night shifts will testify, the body has its own internal clock which weinterfere with at our peril. As each hour of the day clicks by, our body goes through changes; many are internal and regulated by hormones, others are forced by the daily cycles of the modern environment. Balancing them is not always easy.

04.00 We are more likely to wake at this point in the night than any other. We are at one of the most unstable points of the sleep cycle, moving between different phases. During sleep the body produces muramyl peptides, the hormones which renew body tissue and stimulate the immune system. If we lose sleep, we can make up for one-third of it on subsequent nights.

07.00 Levels of serotonin started to rise a couple of hours ago. Serotonin is analogous to adrenalin. But while adrenalin helps us survive extreme changes in the environment, serotonin helps us gear up for the change in pace at the start of the day. Levels of cortisol, a hormone that speeds up the release of glucose into the blood, peak as we wake up.

08.00 Getting up is one of the most physical, arduous and potentially dangerous points in the day. Unsurprisingly, Monday mornings are the most perilous. According to doctors in Germany, the first morning of the week can spark off a whole range of symptoms, including a rise in blood pressure, headaches and stress-induced nausea. At the more extreme end of the scale, Monday mornings see a significant rise in suicides and heart attacks. We are 40 per cent more likely to suffer a coronary on a Monday morning than at any other time of the week. The sudden change in activity after waking up could be one reason, but some doctors blame weekend drinking which raises blood pressure, contributing to the Monday morning heart attack.

Waking up early on a Monday can also come as a jolt to our body clock after a weekend where it has been allowed to settle into its own rhythm. This can have an effect on mood, which may contribute to the increase in suicides.

Swedish research has confirmed the theory that our body clocks work in a 25-hour cycle rather than a 24-hour one, and that rising before 9am can knock the body out of sync, depriving us of our best sleeping hours. Going to bed earlier does not compensate for this.

09.00 The chances of asthma sufferers experiencing an attack rise significantly. This can be attributed to levels of serotonin, but exposure to pollution is also a possible factor.

09.30 Some suggest this is the best time to visit the dentist, as endorphins (the body's natural painkillers) are at their highest; others, however, suggest mid-afternoon may be preferable.

13.00-16.00 Adrenalin levels and body temperature fall. It used to be thought this was caused by "dumping", whereby blood would be diverted to the task of digesting lunchtime food in the stomach. That theory is largely discredited now, and the latest craze to beat the mid-afternoon slump is "power napping". This accords with the more popular theory that our body clocks are programmed for two periods of sleep, one at night and a Mediterranean-style siesta in the afternoon.

This is peak time for motoring accidents, a significant proportion of which are caused by drivers suffering from a post-lunch slump. Some 28 per cent of accidents are caused by tiredness. In one study, researchers at the Sleep Research Laboratory at Loughborough University confirmed that sleep-related accidents peak during mid-afternoon when traffic is at its lightest. The researchers say drivers should plan journeys to avoid travelling at this time of day.

18.00 The body starts to manufacture melatonin, making us lethargic and slowly preparing us for sleep.

19.00 Levels of pain-blocking endorphins and cortisteroids start to fall.

20.00 Statistically, this is a peak time of day to commit suicide - and also the best time of day for a drink. Enzymes in the liver, which can help to break down alcohol, are at their highest now but will gradually fall away over the next three to four hours.

23.00 While experts recommend we should not go to bed before we feel tired, the body is now producing sleep-inducing hormones. Feelings of fatigue can be ignored (often with the help of caffeine) but this knocks our body clocks out of kilter.