The first four stages are considered to be Non-Rapid Eye Movement sleep (NREM) or orthodox sleep. The function of these stages is to restore and rebuild the body after a long period of wakefulness. Vegetative functions dominate NREM sleep. The body temperature, heart rate and blood pressure decrease, muscles relax, and the body metabolism slows.
Stage I. Stage I is a transition between sleep and wakefulness which is usually only five minutes in duration. Short dreams may occur, usually involving images remembered from throughout the day. The brain’s electrical activity slows as exhibited by beta-rhythms on the EEG. The initial stage of Stage I sleep is the one I trouble the most with, usually spending an hour trying to fall asleep. However, I spend, an average of about 30% of my sleep in this stage. Since my recent uptake of marijuana, the initial stage of sleep comes instantly to me and I have no more trouble falling asleep.
Stage II. Stage II is a somewhat deeper level of sleep, characterized by slower breathing and heart rates. The EEG of stage II shows slow beta-rhythms, interspersed with periods of fast alpha-rhythms called sleep spindles and some delta-rhythms.
Stages III and IV. Stages III and IV are the deepest levels of sleep and have the slowest waves as measured by EEG: Stage III has both theta and delta rhythms, while Stage IV has only delta-rhythms. The body uses this time to maintain and restore itself. Growth hormone secretions are at their highest during these stages. Stages III and IV begin after one has been asleep for approximately half an hour. The first episodes of Stage II and IV sleep are usually the longest of the night. As successive cycles of sleep pass, these stages are replaced by longer periods of Stage V sleep.
NREM Sleep The first four stages are considered to be Non-Rapid Eye Movement sleep