A little more detail
The time from lights out to the first epoch of sleep. A National Sleep Foundation expert panel counted 30 minutes or less as one of the indicators of good sleep quality. Daytime sleepiness is assessed with the Multiple Sleep Latency Test (MSLT), in which the same measure is repeated during the day; a short mean latency supports pathological sleepiness.
Source: Ohayon M, Wickwire EM, Hirshkowitz M et al. National Sleep Foundation's sleep quality recommendations: first report. Sleep Health. 2017;3(1):6-19. doi:10.1016/j.sleh.2016.11.006