Sleep Cycle

Sleep stages each last 90 to 120 minutes. Though dreams can occur in any of the sleep stages, the most memorable and vivid dreams occur during REM sleep in the fourth and final stage. After a full cycle is completed, the cycle repeats itself, which happens an average of five to seven times each night. As the cycles repeat, more and more time is spent during the fourth stage.

Stage 1:

• Entering a light sleep
• Muscle relaxation
• Lowered body temperature
• Slowed heart rate

Stage 2:

• Further drop in body temperature
• Muscles more relaxed
• Immune system begins working to repair bodily damage
• Endocrine glands secrete hormones
• Blood pumped to muscles for reconditioning

Stage 3:

• Even deeper sleep
• Metabolic levels extremely slow

Stage 4:

• REM sleep (rapid eye movement)
• Occurs 90-100 minutes after onset of sleep
• Blood pressure rises
• Heart rate speeds up
• Respiration erratic
• Brain activity increases
• Involuntary muscles paralyzed
• Dreams usually occur

-Source