Sleep Onset Latency

Sleep onset latency, often abbreviated SOL, is the time it takes to fall asleep after beginning to try to sleep. It is usually expressed in minutes. The starting point matters: getting into bed is not necessarily the same as attempting to sleep, so time spent in bed before that attempt is separate from sleep onset latency .¹

Sleep onset latency describes the beginning of a sleep period. It differs from total sleep time, which describes how much sleep occurs, and from wake after sleep onset, which describes time spent awake after initially falling asleep. These measures capture different aspects of sleep, so the time needed to fall asleep does not by itself describe the rest of the night .¹

The measurement can come from a person's estimate recorded in a sleep diary or from physiological recordings during a sleep test. A diary captures the person's recalled experience. In standardized daytime sleep testing, latency is measured from lights out to the first recorded interval classified as sleep using brain activity and other signals. The method and starting point therefore need to be clear when interpreting a reported value .^1,2

Sleep latency also helps assess the tendency to fall asleep during the day. In a multiple sleep latency test, measurements from several scheduled nap opportunities are averaged. Shorter latencies in this controlled setting indicate greater daytime sleepiness, but prior sleep, the timing of testing, medications, and testing conditions can affect the result .²