Underlying differences in biological clocks mean that some people need a lot more sleep than others.
It's long been a puzzle as to why some people need at least nine hours sleep while others get by on less than six. The average amount of sleep an adult gets is about seven and a half hours a night.

Now researchers at the National Institute of Mental Health in Bethesda, Maryland, in the US, have found that there is an underlying biological reason for the differences in sleep needs between individuals. They studied a group of both long and short sleepers in a sleep lab, measuring hormone levels, temperature and other physiological markers. This indicated the length of the circadian cycle - the internal clock that controls our sleep and wake cycles. The long sleepers had a longer biological sleep time than the short sleepers. So it is biology that drives the need for a longer night's sleep - not habit, or laziness. This new research should encourage people to seek out and follow their natural sleep patterns.