How is sleep efficiency defined?

Sleep efficiency is defined as the total sleep time divided by the total time spent in bed, typically expressed as a percentage. This measurement reflects how effectively an individual is able to sleep during the time they are in bed, indicating the quality of sleep.

Option A accurately describes this definition. By providing the ratio of total sleep time to the total duration of time in bed, it offers a clear measure of how efficiently sleep is utilized. Higher sleep efficiency percentages indicate that more time in bed is spent sleeping rather than awake, which is a key indicator of good sleep quality.

While options C and D provide information about specific aspects of sleep, they do not address the broader concept of sleep efficiency as defined. Option C focuses solely on REM sleep percentage, and option D discusses deep sleep duration rather than overall time management during sleep. Thus, the appropriate answer in this context involves the total sleep time relative to the total time spent in bed.