In sleep science, sleep onset latency (SOL) is the length of time that it takes to accomplish the transition from full wakefulness to sleep, normally to the lightest sleep stage non-REM.1
Daytime tests, such as the MSLT, measure sleep onset latency as an indication of excessive daytime sleepiness (EDS). A sleep onset latency of 15-25 minutes in a person's biological daytime is considered normal. A shorter latency is likely a sign that a person suffers from sleep deprivation. A sleep onset latency of less than five minutes indicates either very unhealthy daytime sleepiness or one of several sleep disorders, for example narcolepsy.
A long SOL at bedtime can be an indication of a sleep disorder, including but not limited to insomnia.
