What does "Sleep Efficiency" measure?

Sleep efficiency measures the ratio of total sleep time to the total time spent in bed. This metric is an important indicator in sleep studies, as it assesses how effectively an individual is able to sleep during the period they are in bed. A higher percentage of sleep efficiency suggests that more of the time in bed is spent actually sleeping, which is generally a sign of good sleep quality.

When considering the other options, they focus on different aspects of sleep rather than the efficiency of sleep itself. The total number of hours spent in bed does not provide insight into how much of that time is spent sleeping. The duration of deep sleep phases highlights a specific stage of sleep but does not address the overall ratio of sleep time. Finally, the quality of REM sleep, while important for understanding sleep architecture, is also not related to the concept of sleep efficiency. Thus, the focus on the ratio of total sleep time to time in bed distinguishes sleep efficiency as a measure of overall sleep effectiveness.