Data collected from EEGs (electroencephalograms) of brain activity during sleep have shown at least four separate stages of sleep. During stage 1, heart and breathing rates decrease slightly, the eyes roll slowly from side to side, and an individual experiences a floating sensation. Stage 1 sleep is not usually classified as “true” sleep. This stage generally lasts only five minutes. Individuals awakened during stage 1 sleep will often insist that they were not sleeping, but merely “resting their eyes.” Stage 2 sleep is characterized by the appearance of short bursts of waves known as “sleep spindles” along with “K complexes,” which are high-voltage bursts that occur before and after a sleep spindle. Eyes are generally still and heart and breathing rates decrease only slightly. Sleep is not deep. Stage 3 sleep is intermediate sleep and is characterized by steady, slow breathing, a slow pulse rate, and a decline in temperature and blood pressure. Only a loud noise awakens sleepers in stage 3 sleep. Stage 4 sleep, known as oblivious sleep, is the deepest stage. It usually does not begin until about an hour after falling asleep. Brain waves become even slower, and heart and breathing rates drop to 20 or 30 percent below those in the waking state. The sleeping individual in stage 4 sleep is not awakened by external stimuli, such as noise, although an EEG will indicate that the brain acknowledges such stimuli. Stage 4 sleep continues for close to an hour, after which the sleeper will gradually drift back into stage 3 sleep, followed by stages 2 and then 1, before the cycle begins again.
What are the stages of sleep?
