Some of the world's greatest leaders, from Napoleon Bonaparte and Winston Churchill to Bill Clinton and Margaret Thatcher, are said to have managed on just four to six hours' sleep a night, whereas the typical teenager finds it difficult to get out of bed in less than 10.

    Newborn babies can sleep for up to 18 hours - admittedly at irregular intervals - whereas an elderly person may find it hard to sleep longer than six, although they often have to resort to the odd afternoon nap to make up for what they lacked at night.

    Sleep is the quintessential ingredient of life. Every animal does it at some point in the 24-hour cycle and people who are forcibly deprived of sleep are effectively undergoing torture. But the big unanswered question is how much sleep do we actually need?

    Some people seem happy with four or five hours, although most people would feel sleep-deprived on less than six. Others need a good seven or eight hours of sleep and adolescents are renowned for extended kips.

    So how much sleep is necessary for a healthy mind and body, and does this amount truly need to vary between people and age groups?

    The latest study into sleep may help to resolve the issue with the discovery that certain people in the population carry the smallest of genetic mutations in a gene that appears to play a significant role in deciding just how much sleep human beings need.

    Scientists studied an extended family in California and found that a mother and her daughter shared a life-long habit of rising in the very early hours of the morning with no apparent ill-effects. They routinely went to bed between 10.30pm and 11pm and got up between 4am and 4.30am.

    The researchers took blood samples from all members of the family and analysed their DNA for any signs that could explain this unusual behaviour. The tests revealed that the mother and her daughter did in fact share a tiny "point mutation" in a gene known as hDEC2, which is known to affect the regulation of other genes and has been implicated in the control of sleeping patterns in animals.

    一些世界頂級領袖，從拿破侖·邦那巴，溫斯頓·丘吉爾到比爾·克林頓再到瑪格萊特·撒切爾，據說他們每天晚上僅需4到6個小時的睡眠時間，相比典型的青少年卻覺得不夠十小時就要爬起床真是難啊。

    新生兒一天睡眠達到18個小時之多--當然期間有不規律的間斷。而老年人盡管他們時常得睡個額外的午覺來彌補夜間的睡眠不足，他們仍會覺得很難一天睡得超過6個小時。

    睡眠是生命不可或缺的重要養分。每種動物都要在24小時的周期內睡上一段時間，被強迫性剝奪睡眠的人肯定甚感折磨。但是，不得而解的大謎團是，我們究竟需要多少睡眠時間？

    有些人只需四五個小時就心滿意足了，雖然大多數人會覺得少于六小時簡直就是被剝奪了睡眠的權利。有些人則需要七八小時的質量好的睡眠。青少年則是以睡懶覺出了名的。

    那么為了身心健康，多少睡眠才合適呢？是不是不同的人或不同的年齡段的人對睡眠時間的要求也不一樣呢？

    最新的研究可能有助于解決這個疑問。研究發現，有部分人的基因攜帶了最小的基因突變，而這在決定人類究竟需要多少睡眠的問題上起了關鍵的作用。

    科學家對加利福尼亞的一個大家庭做了研究，他們發現有一對母女都有一個一輩子的習慣，她們清晨很早的時侯就起床了，沒有什么明顯的不良反應。這對母女日如一日地在晚上十點半至十一點就寢，次日四點至四點半起床。

    研究人員對這個家庭的所有成員抽了血樣來分析他們的DNA以尋出任何可能解釋這個不尋常之舉的體征。這項測試發現，這對母女的基因里都攜帶了一個細小的"點突變",即hDEC2.眾所周知，這個點突變影響了其他基因組織，從而參與決定了動物的睡眠類型。