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.

    一些世界顶级领袖，从拿破仑·邦那巴，温斯顿·丘吉尔到比尔·克林顿再到玛格莱特·撒切尔，据说他们每天晚上仅需4到6个小时的睡眠时间，相比典型的青少年却觉得不够十小时就要爬起床真是难啊。

    新生儿一天睡眠达到18个小时之多--当然期间有不规律的间断。而老年人尽管他们时常得睡个额外的午觉来弥补夜间的睡眠不足，他们仍会觉得很难一天睡得超过6个小时。

    睡眠是生命不可或缺的重要养分。每种动物都要在24小时的周期内睡上一段时间，被强迫性剥夺睡眠的人肯定甚感折磨。但是，不得而解的大谜团是，我们究竟需要多少睡眠时间？

    有些人只需四五个小时就心满意足了，虽然大多数人会觉得少于六小时简直就是被剥夺了睡眠的权利。有些人则需要七八小时的质量好的睡眠。青少年则是以睡懒觉出了名的。

    那么为了身心健康，多少睡眠才合适呢？是不是不同的人或不同的年龄段的人对睡眠时间的要求也不一样呢？

    最新的研究可能有助于解决这个疑问。研究发现，有部分人的基因携带了最小的基因突变，而这在决定人类究竟需要多少睡眠的问题上起了关键的作用。

    科学家对加利福尼亚的一个大家庭做了研究，他们发现有一对母女都有一个一辈子的习惯，她们清晨很早的时侯就起床了，没有什么明显的不良反应。这对母女日如一日地在晚上十点半至十一点就寝，次日四点至四点半起床。

    研究人员对这个家庭的所有成员抽了血样来分析他们的DNA以寻出任何可能解释这个不寻常之举的体征。这项测试发现，这对母女的基因里都携带了一个细小的"点突变",即hDEC2.众所周知，这个点突变影响了其他基因组织，从而参与决定了动物的睡眠类型。