Sleep and General Anesthesia as Altered States of Consciousness
Munglani R, Jones JG; Journal of Psychopharmacology 6:399-409, 1992.
Stages of Sleep
EEG recordings show that we go through five stages of sleep each with its characteristic brain-wave activity. Stage 1 is the transition stage from wakefulness to sleep and is identified with theta waves and last between 1 to 7 minutes. In stage 2 EEG recordings show fast-frequency burst of activity called sleep spindles. In stages 2 through 4 muscle tension, heart rate, respiration, and temperature gradually decline, and it becomes more difficult to be awakened. 30 minutes after falling a sleep we pass through stage 3 and enter stage 4. In this stage EEG recordings show delta waves and it is the deepest stage of sleep. There is a marked secretion of growth hormone in stage 4. Sleep researchers determine what sleep stage a person is in by the ratio between the number of sleep spindles and the number of delta waves (Munglani and Jones, 1992). After this stage we go back to stage 2 and then enter REM sleep or rapid eye movement sleep. Here EEG tracings look exactly like the beta waves that are observed when we are completely awake. In fact, brain-imaging studies show that the neurons in the cerebral cortex become much more active during REM sleep (Munglani and Jones, 1992). REM sleep makes up 20% of our sleep time and during this stage we experience vivid dreams. We go through this sleep cycle 5-6 times during 8 hours of sleep (Plotnik, 1993).
Web Resource: philpapers.org
 Gokalp, G, 'Sleep and Dreams'. Internet Reference, 1999
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