The first thing to recognize about the stages of sleep is that they are not actually linear; that is to say, a person's sleep cycles through the stages several times in any given night. The first stage is one of very light sleep, so light in fact, that the person might think they're awake. This stage is seen often by people who think they're awake all night, but in fact they probably dozed in this stage off and on. It is not good sleep, nor restful sleep, but it is a transitional sleep. The second stage is roughly twenty minutes of time when the body's temperature and heart rate are declining; the person then enters another transitional phase that usually leads to about thirty minutes of deep sleep, followed by what is sometimes called REM sleep, which is known to produce the most restful sleep. Ironically enough, during REM sleep, one's muscles relax, but brain activity speeds up. These cycles, as noted above, are not necessarily experienced in a linear manner; after the first stage, one generally moves through stages 2-5, then back and forth between those same stages throughout the night.
Many things, of course, can impact one's sleep patterns and cause mild or severe insomnia. The use of alcohol and drugs can impact sleep quality; additionally, millions of Americans use specific drugs aimed at helping them fall asleep. Benadryl and melatonin are frequently used over the counter remedies, while more chronic insomnia sufferers may turn to stronger prescription drugs such as Ambien.