A

Babies sleep about eighteen hours a day． Mrs Thatcher and Napoleon both said they only needed to do it three or four hours a night． Sleep，no one can live without it． But how much do we really need?

Research(研究) by the National Sleep Foundation in Washington says that we all need eight hours-' sleep every right． Scientists -have found that people who sleep for less than six and a half hours a night are more often ill than people who sleep for eight hours． Going without sleep also increases(增加) the chance of serious illness． "Workaholics(l作狂)  who sleep for less than five hours often die young，and do less well at work．

The scientists found that adults usually sleep for seven hours a night，with 32% sleeping less than six hours．

It also says that the idea that we need less sleep as we get older is completely untrue．

"People have no idea how important sleep is to their lives，"Dr． Thomas Roth，director of the Foundation says，"Good health needs good sleep． "

"But not too much of it，"says Professor Jim Home of Loughborough University，"sleep is like food and drink，"he believes，"you would always like to have a little bit more，but that doesn’t mean you need it． "Professor Home studied a group of people who could spend as many hours as they wanted in bed，after ten hours they didn't find it any easier to get up in the morn-ing． And people who sleep for more than nine hours a night die younger than people who usual-ly sleep for seven or eight!

(    )21． How many hours a day do babies sleep?