Why is it so difficult to fall asleep when you are overtired? There is no one answer that applies to every individual. But many people fail to note the distinction between fatigue -- physical tiredness -- and sleepiness, the inability to stay awake. It's possible to feel "tired" physically and still be unable to fall asleep, because while your body may be exhausted, you don't feel sleepy. To fall asleep, you need adequate time to unwind, even if you feel fatigued. It's not so easy to simply "turn off."

According to Carl E. Hunt, director of the National Center on Sleep Disorders Research in Bethesda, Maryland, most people do not allow themselves sufficient deceleration.

Lack of sleep complicates matters even more. Experts say adults need at least 7 to 8 hours of sleep a night to function properly. When you get less sleep than that on consecutive nights, you begin to accrue "sleep debt." As sleep debt increases (and functionality decreases), your body experiences a stress response and begins to release adrenaline(= adrenalin[生化]肾上腺素). Now a vicious cycle has been created: You experience the feeling of being more and more tired, but your body is increasingly stimulated. "Power sleeping" for more hours on weekends is only a temporary solution. "There is no substitute for getting a good night's sleep on a regular basis," says Hunt.

Most of us, however, don't get the sleep we need. According to the 2002 National Sleep Foundation, Americans sleep an average of 6.9 hours per night during the week, and 58 percent of adults experience symptoms of insomnia a few nights a week or more.

为何你劳累过度却又如此难以入睡？这并没有一个可适用于每一个人的统一答案。但很多人都未注意到疲倦(身体上的劳累)和困倦(无法保持清醒状态)之间的区别。身体上感到“劳累”却仍然无法入睡是可能的，因为你的身体可能感到疲惫，但你不觉得困倦。要入睡，你得有充足的时间来放松，即便你已感到疲倦。要想一下就“转而入睡”，没那么容易。

马里兰州贝塞斯达全国睡眠紊乱研究中心主任卡尔·E·亨特说，大多数人都没有给自己充分的舒缓时间。

缺少睡眠使事情变得更为复杂。专家们说，要使身体机能运转正常，成人每晚至少需要7～8个小时的睡眠。如果你连续数晚睡眠都少于这个时间，你就要开始负上“睡眠债”。当睡眠债越积越多(同时身体机能减退)，你的身体将做出压力反应，开始释放肾上腺素。这样就形成了一个恶性循环：你感觉越来越累，但你的身体受到的刺激却越来越强。周末猛补几个小时的觉只是权宜之计。“有规律地每晚睡一个好觉，这是不可替代的，”亨特说。

然而，我们当中大多数人并未睡足我们所需的觉。根据2002年全国睡眠基金会的数据，美国人一周内平均每晚睡眠为6.9个小时，58%的成年人在一周或更长的时间内总有几晚有失眠症状。