One widely dispersed recommendation made by sleep specialists is that we need at least eight hours of sleep per night to stay healthy.

This recommendation is substantiated in large part on a single study, in which young, healthy individuals were required to spend up to twelve hours per day in bed for almost a month.

Subsequent to a few days, their sleep duration stabilized at about eight and one quarter hours per night.

Some sleep researchers have decided from these findings that are biologically programmed to sleep over eight hours per night. Nevertheless, not all sleep researchers agree with this conclusion.

Some scientists have suggested that sleeping less than eight hours per night is a significant risk factor for increased sickness, and even death.

It was compared physiological functioning in healthy, young male sleepers who were allowed to sleep just under four hours a night, just over seven hours and just over nine hours over several weeks.

It was found that, compared to nine hours of sleep, four hours of sleep impaired the body's ability to process sugar (meaning that people could become more susceptible to diabetes or obesity).
But, these generalizations are premature.

Firstly, it was only found changes in physiological activity when four hours of sleep was compared to nine hours of sleep. It may be an unrealistic comparison since very few people can, or do, sleep nine hours per night.

Moreover, there were no changes in physiological activity when four hours of sleep was compared to a more realistic seven hours of sleep. Additionally, only young, healthy sleepers were used (so the results may only apply to young people), and the same physiological changes have been observed in response to daily life stress.

Insomniacs (who sleep less than six hours per night on average) do not have a greater incidence of diabetes or obesity. And other well-done studies have shown that sleep deprivation does not result in significant changes in metabolism or physiological functioning.