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]腎上腺素). 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.

    為何你勞累過度卻又如此難以入睡？這并沒有一個可適用于每一個人的統一答案。但很多人都未注意到疲倦(身體上的勞累)和困倦(無法保持清醒狀態)之間的區別。身體上感到“勞累”卻仍然無法入睡是可能的，因為你的身體可能感到疲憊，但你不覺得困倦。要入睡，你得有充足的時間來放松，即便你已感到疲倦。要想一下就“轉而入睡”，沒那么容易。

    馬里蘭州貝塞斯達全國睡眠紊亂研究中心主任卡爾·E·亨特說，大多數人都沒有給自己充分的舒緩時間。

    缺少睡眠使事情變得更為復雜。專家們說，要使身體機能運轉正常，成人每晚至少需要7～8個小時的睡眠。如果你連續數晚睡眠都少于這個時間，你就要開始負上“睡眠債”。當睡眠債越積越多(同時身體機能減退)，你的身體將做出壓力反應，開始釋放腎上腺素。這樣就形成了一個惡性循環：你感覺越來越累，但你的身體受到的刺激卻越來越強。周末猛補幾個小時的覺只是權宜之計。“有規律地每晚睡一個好覺，這是不可替代的，”亨特說。

    然而，我們當中大多數人并未睡足我們所需的覺。根據2002年全國睡眠基金會的數據，美國人一周內平均每晚睡眠為6.9個小時，58%的成年人在一周或更長的時間內總有幾晚有失眠癥狀。