Sunset
Things like heart rate, metabolic rate, temperature, alertness and urinary output all rise in the morning, peak in late afternoon and drop at night - regardless of external cues such as changing light or regular mealtimes.
However there is variation in our individual clocks, which appears to be genetic. And while most of us are mentally and physically most alert during the main part of the day, 5% are classified by sleep experts as having extreme morning or evening preference - in other words we are 'larks' or 'owls'.
Sleep researchers have now identified around 12 genes that appear to influence morning or evening preference. On the whole women are more larkish than men and we all become increasingly lark-like as we get older. It is thought that this could be due to age-related changes in the production of hormones such as melatonin or a difference in the way genes work in later life.
The right time
6-7am
Body temperature rises, along with levels of endorphins (the body's natural painkillers). The stress hormone cortisol circulates, triggering the release of glucose into the blood to provide you with energy for the morning ahead.
Best time to: Make love as sex hormones oestrogen and testosterone are at their peak.
10-11am
Peak of mental alertness.
Best time to: Do mental work that requires concentration or do the crossword!
12 noon
Lowest energy point. Temporary dip in adrenaline and body temperature.
Best time to: Have a nap.
3pm-5pm
Body temperature and adrenalin rise, cortisol peaks. Optimum period of combined mental/physical functioning. Most relaxed period.
Best time to: Visit the dentist, as our pain threshold is at its highest.
5-7pm
Coordination, stamina, body temperature and adrenaline are at their peak.
Best time to: Take a spot of exercise.
9pm
Body begins to produce additional melatonin, in preparation for sleep.
Best time to: Watch a non-demanding programme on TV.
11pm
Our system quietly shuts down, minimising the need for food, drink and visits to the toilet through the night.
Best time to: Have a bath and get ready for bed.
3-5am
Body temperature and hormones are at their lowest, to aid deep slumber.
Best time to: Sleep