Sleep Efficiency is calculated by dividing Total Sleep Time by Total Time in bed. "Normal" is considered to be 80% or greater. For example, if a person spends 8 hours in bed (from 10 p.m. to 6 a.m), at least 6.4 hours or more should be spent sleeping to achieve an 80% or greater sleep efficiency.  Most healthy and young adults have sleep efficiencies above 90%.