When you are awake and alert, your brain waves are fast. After about five minutes of this alpha state, you slip into a semiconscious state called Stage 1 sleep. Stage 1 sleep usually lasts only a few minutes, and it is marked by relaxation of the muscles, slower breathing and a reduction in pulse rate.
Brain waves slow even further.
For someone who is not sleep-deprived, Stage 2 usually lasts 15 to 30 minutes. Stages 3 and 4 normally last a total of 30 to 40 minutes on your first sleep cycle.