What's the circadian rhythm?

The circadian rhythm runs roughly to a 24-hour cycle, during which there are two periods of sleepiness: one is at its greatest between 11 pm and 5 am, the other occurs briefly around 2–3 pm.

You get your best sleep during the long period of natural sleepiness at night when the hormones of wakefulness – corticosteroids and adrenaline – are at their lowest levels and darkness stimulates melatonin production.

After 5 am stimulating hormones start to wake you up, which is why sleeping during the day is often unrefreshing. Shift work and jet lag, and other external stimuli such as all-night parties, can force your body to over-ride this natural rhythm so that you feel tired while you are awake and restless while you are asleep.