Phases of Sleep
Stage 1: Light sleep. Slow eye movement and reduced muscle activity. (Forms about 5% of total sleep)
Stage 2: Eye movement stops and brain waves become slower, with occasional bursts of rapid waves called sleep spindles. (Forms about 55% of total sleep)
Stage 3: Extremely slow, delta waves begin to appear, interspersed with smaller, faster waves. (Forms about 5% of total sleep)
Stage 4: The brain produces delta waves almost exclusively. It is difficult to wake someone during stages 3 and 4, known as deep sleep. There is no eye movement or muscle activity. People awakened while in deep sleep do not adjust immediately and often feel disoriented for several minutes after waking up. (Forms about 15% of total sleep)
Stage 5: The rapid eye movement (REM) stage. Breathing becomes more rapid, irregular, and shallow. Eyes jerk rapidly in various directions and limb muscles become temporarily paralysed. Heart rate increases, blood pressure rises, and males develop penile erections. When people awaken during REM sleep, they often describe bizarre and illogical tales — dreams. (Forms about 25% of total sleep)