REM rebound is lengthening and increasing frequency and depth of REM sleep which occurs after periods of sleep deprivation. When people have been prevented from experiencing REM, they take less time than usual to attain the REM state. When people are unable to obtain an adequate amount of REM sleep, the pressure to obtain REM sleep builds up. When the subject is able to sleep, they will spend a higher percentage of the night in REM sleep.
Polyphasic schedules, which reduce sleep hours, often rely on the SOREM mechanism, which allows to enter REM soon after sleep onset.