Always tired, no matter how much you sleep? Discover the hidden causes of chronic fatigue — from sleep apnea to vitamin ...

Just 48 hours without sleep is considered extreme sleep deprivation, but you'll probably get weird before then. After 18 hours with no sleep, you might feel a little drunk. Staying awake that long ...

And that's no bad thing because ... For the next three nights, however, we restricted their sleep to just four hours. Getty Images Each day our volunteers filled in a psychological questionnaire ...