Sleep efficiency
How much of your time in bed you actually spend asleep. If you lie in bed for eight hours and sleep for six, the ratio is three quarters.
A little more detail
Total sleep time as a percentage of time in bed (or recording time). In sleep restriction therapy, time in bed is usually adjusted against an 85% threshold; the definition of the denominator differs between studies.
Source: Reed DL, Sacco WP. Measuring sleep efficiency: what should the denominator be? J Clin Sleep Med. 2016;12(2):263-266. doi:10.5664/jcsm.5498