Teenagers need as much sleep as small children (about 10 hrs) while those over 65 need the least of all (about six hours). For the average adult aged 25-55, eight hours is considered optimal.
 

To drop off we must cool off; body temperature and the brain's sleep-wake cycle are closely linked. That's why hot summer nights can cause a restless sleep. The blood flow mechanism that transfers core body heat to the skin works best between 18 and 30 degrees. But later in life, the comfort zone shrinks to between 23 and 25 degrees - one reason why older people have more sleep disorders.
 

REM sleep may help developing brains mature. Premature babies have 75 per cent REM sleep, 10 per cent more than full-term bubs. Similarly, a newborn kitten puppy rat or hamster experiences only REM sleep, while a newborn guinea pig (which is much more developed at birth) has almost no REM sleep at all.
 

The record for the longest period without sleep is 18 days, 21 hours, 40 minutes during a rocking chair marathon. The record holder reported hallucinations, paranoia, blurred vision, slurred speech and memory and concentration lapses.
 

Exposure to noise at night can suppress immune function even if the sleeper doesn't wake. Unfamiliar noise, and noise during the first and last two hours of sleep, has the greatest disruptive effect on the sleep cycle.
 

The "natural alarm clock" which enables some people to wake up more or less when they want to is caused by a burst of the stress hormone adrenocorticotropin. Researchers say this reflects an unconscious anticipation of the stress of waking up.

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