The infant child sleeps much longer than an adult. In the first year of life a child will typically spend about 16 hours asleep, half of this being in REM. By the age of one this has dropped to about 12 hours of total sleep with about four hours of REM. Lots of sleep in early life does seem to tie in with restoration theory, since this is a time of rapid growth both in the body and in the brain. Early life is a steep learning curve so presumably the brain is working over time to assimilate all this new information by making new and ever more complex interconnections. According to Oswald this would be facilitated by plenty of REM sleep.
Evolutionary sleep theorists suggest all this infant sleep is designed to take the pressure off of parents who can get on with essential chores such as finding food.
By adolescence hormones seem to be playing an ever-increasing role in the sleep pattern. Hormone production at night is disturbing sleep and leading to sleep deprivation. Studies suggest that adolescents need more sleep that pre-adolescents not less. However, schools usually expect the older age group to start earlier than the younger age group. Recent research is suggesting that many adolescents have DSPS (mentioned in biorhythms) that results in later sleep onset and difficulty waking in the morning. As a result some schools are now experimenting with a later start to the school day and are reporting improved performance and results.
By the time we have reached maturity we usually sleep for 8 hours with only one quarter (2 hours) being spent in REM. Note, people who sleep longer tend to spend much of the extra time in REM. As a species, in the West we sleep less than we did a century ago. It is estimated that in the UK we now spend only 7.5 hours asleep per night compared with 9 hours in Victorian times.
Kripke et al (2002) report that sleeping longer is correlated with ill-health. In a huge survey of over one million adults they found that those sleeping six or seven hours have a greater life expectancy than those sleeping eight hours longer. However, you have probably noticed the weak link in this argument… the word ‘correlated!’ It would seem likely that people who are ill may need to sleep longer, so underlying health problems are causing the increase in mortality and the increased need for sleep.
The wrinkly years
As we get older still there are further changes. REM continues to decrease, and by the time we reach 60, stage 4 is non-existent. As a result older people are more easily awoken and often complain of insomnia. This loss of deep sleep may explain the deterioration seen in later life. No deep sleep, no growth hormone for repairs. As a result there is increased loss of muscle tone, lack of energy and increased risk of osteoporosis as bone density declines.