Some people need more sleep than others

A team from the University of Surrey said the "clock gene" called Period 3 is responsible. The same gene dictates if someone is an "owl" or a "lark".

Most people need around eight hours sleep a night - but famous exceptions such as former Prime Minister Margaret Thatcher have survived on much less.

The research is published in the journal Current Biology.

There are two variants of the Period 3 gene found in the human population, with either long or short versions of the corresponding protein.

Everyone has two copies of the gene, either of which might be the long or short form.

Slow-wave sleep

The University of Surrey team looked at how 24 people with only the longer gene variant and those with only the shorter one coped with being kept awake for 48 hours in a lab.

Margaret Thatcher was said to need just four hours sleep a night

The researchers found that although some participants struggled to stay awake, others had no problems doing so.

Performance in tasks, such as attention and reaction tests, was also checked throughout the period.

The results were most pronounced during the early hours of the morning - between 4 and 8 am - when peoples with the longer variant of the gene performed very poorly on tests for attention and working memory.

This is the part of the night when shift-workers have most difficulty staying awake, and when many accidents related to sleepiness occur.

The researchers also found that when the study participants were allowed to sleep normally, those possessing only the longer form of the gene spent about 50% more of their time in slow-wave sleep, the deepest form of sleep - and a key sign someone needs more sleep.

Writing in Current Biology, the researchers said the study implied that having the variant "may be an important marker for individual differences in sleep and susceptibility to sleep loss, which are major causes of health problems and accidents in our society."