Sleep Efficiency (or Sleep-Efficiency Index):
The proportion of sleep in the episode potentially filled by sleep (i.e., the ration of total sleep time to time in bed) or Sleep Efficiency = Total sleep time multiplied by time in bed. (JSD); Normal sleep efficiency is at least 85% (asleep 85% of the night). It is reduced in a number of situations, such as insomnia or simply lab effect. (Sleepydave) From the American Sleep Apnea Association.