As well as sleep itself being important, other aspects of the familiar 24-hour cycle can be important in many ways.
Sleep cool. Duvets are not adjustable so cotton sheets (no electric charges we get from synthetics) and as many light woollen blankets as you need to keep just warm enough for comfort. Sleeping cool will stop dry, itchy skin and can eliminate a major cause of cracked heels. 
Leukaemia - Exposure to light at night may be implicated in the 50 per cent increase in leukaemia in children under five since the 1950s. Professor Russel Reiter, of the University of Texas said in London in early September 2004 that exposure to light at night disrupts the body's circadian rhythms and suppresses nocturnal production of the hormone melatonin. "As an antioxidant, in many studies melatonin has been shown to protect DNA from oxidative damage. Once damaged, DNA may mutate and carcinogenesis may occur" he said. 
Are you getting enough sleep? - If you are not being kept awake by television, the internet or some other screen, coffee or non-palaeolithic cause and you are not using an alarm or other external means of waking up refreshed at a convenient time in the morning, you are probably getting adequate sleep. Getting adequate sleep tells your body that all is well. You're not inadvertently communicating the message to your body that it needs to be alert for predators or out scavenging. Staying up late sends this message of searching - unsuccessfully - for food. Sleep, particularly that obtained before midnight, stimulates release of human growth hormone and testosterone and promotes cell repair .
1. Hugh Molloy and Garry Egger, Skin Fitness, 2008, pages 54-65
2. Canberra Times, 13 September 2004
3. Doug McGuff and John Little, Body by Science, 2009, page 200
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Page last up-dated 28 May 2009