Sleep disorders occur in some people with cancer and may be caused by physical illness, pain, treatment drugs, being in the hospital, and emotional stress. Sleep has two phases: rapid eye movement (REM) and non-REM (NREM). REM sleep, also known as "dream sleep," is the phase of sleep in which the brain is active. NREM is the quiet or restful phase of sleep. The stages of sleep occur in a repeated pattern of NREM followed by REM. Each sleep cycle lasts about 90 minutes and is repeated 4 to 6 times during a 7- to 8-hour sleep period. The four major categories of sleep disorders that interfere with normal sleep patterns include:

• The inability to fall asleep and stay asleep (insomnia).
• Disorders of the sleep-wake cycle.
• Disorders associated with sleep stages, or partial waking (parasomnia).
• Excessive sleepiness.

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