Stages 3 and 4 come after this, and are types of deep sleep. Deep sleep is the part of sleep that our body and brain needs to recover from the day. It sometimes called delta sleep, after the delta waves the brain generates. We don't dream during this time. During a normal night, people usually cycle through the various stages several times.
Problems arise when the pattern of cycles is broken.
Researchers at Loughborough University's Sleep Research Laboratory have found that people are designed for 2 sleeps a day - the main one at night, and a nap in the afternoon. This suggests that an afternoon siesta, as people in warmer parts of the world have, may be a physiologically good idea. Those of us who don't have the luxury of an afternoon nap still tend to get sleepy at about this this time. Interestingly, people who increase their night-time sleep find that this afternoon 'dip' disappears. A 10-minute nap at lunchtime is just as effective.