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5) How do researchers know that there are different stages of sleep?

User Dhondup
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stages of sleep and sleep cycles. usually sleepers pass through five stages: 1 2 3 4 and REM(rapid eye movement ) sleep. these stages progress cyclially from one through rem then Begin Again with Stage 1. a complete sleep cycle takes an average of 90 to 110 minutes, with each stage last thing between 5 to 15 minutes.
User Jacek Glen
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By countless hours of research and tests
User Gdaras
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