Home > Encyclopedia of Science > Sleep and Sleep Disorders

• Necessity of sleep
• Purpose of sleep
• Words to Know
• Stages of human sleep
• REM sleep
• How much sleep?
• Insomnia
• Narcolepsy
• Sleep apnea
• Other sleep disorders

Sleep is a normal state of decreased consciousness and lowered metabolism during which the body rests. As a natural, necessary, and daily experience for humans and most other vertebrates (animals that have a backbone or spinal column), sleep has four stages through which we cycle several times a night. A sleep disorder is any condition that interferes with our regular sleep cycle, ranging from insomnia (pronounced...