It may be helpful, therefore, to review what is known about the physiology of sleep, sleep disorders, good sleep habits and advances in the treatment of sleep disorders. However, when researchers in the 1950s took a closer look at the physiological events that occur during sleep, they were able to identify two distinct phases and four stages of sleep. These various states of sleep occur in a relatively predictable time sequence each night and appear to be controlled by different, but linked, neurochemical systems. Stage one, which is a transitional time of relaxed wakefulness, tends to be a short stage, sometimes lasting only a few minutes. Stage two is characterized by deeper sleep.
Brain waves are slow and become almost coma-like.
As people age, the amount of deep sleep they experience decreases and may actually stop altogether. Other physiological changes that occur during non-REM sleep include a decrease in blood pressure, a slowing of the heart rate and a lowering of body temperature. Changes in hormone secretion, stomach acid production and brain temperature also occur during this phase. During the REM phase the eyes move rapidly under closed lids. Most dreaming occurs in this phase of sleep.