An adult with a healthy sleep cycle is most likely to enter rem sleep

T

Explanation: The majority of adults have a Rapid Eye Movement while they sleep. So it happens when they go to bed after 90 minutes generally.

90 minutes after falling sleep.

Explanation:

The REM sleep is a stage in which the body relaxes, the brain  is more active and it is when people have dreams. In this stage, there is a rapid movement of the eyes, there is not muscle activity and people normally enter REM sleep 90 minutes after falling sleep.

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