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

By countless hours of research and tests

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.