Non-REM sleep accounts for about 75 to 80% of your total sleep each night, or about six hours of non-REM sleep if you sleep eight hours in total. You spend the remaining two hours -- roughly 20% of the night -- in REM sleep. It seems that with training, people can gradually reduce their normal amount of sleep by an hour or two without feeling too tired. The main difference between being awake and being asleep is that in the latter state, consciousness does not intervene. When you're asleep, it's somewhat as if your brain were closing itself off and becoming a sort of "temporary autonomous zone". But in reality, your brain is never completely cut off from the rest of the world: even during the deepest stages of sleep, a strong enough stimulus from your environment will be perceived by your brain and may cause you to wake up. On every normal day of our lives, each of us engages in two very distinct kinds of behaviours : sleeping and being awake. On the surface, these two behaviours are so different from each other that even extraterrestrials who came to observe life on Earth would notice the difference right away. The entry in their ship's log might read: "Earthlings spend about two-thirds of each day upright and moving around and the other third lying down and still." But what these extraterrestrials would not notice right away, and what even Earth's own scientists did not begin to discover until the