Wake after sleep onset (WASO)

Wake after sleep onset is the total time a person spends awake between first falling asleep and finally waking up, a core measure of how fragmented sleep is.

WASO is measured in minutes and is derived from polysomnography, actigraphy or a sleep diary. It sits alongside sleep onset latency, which covers the time before first sleep, and total sleep time; together they determine sleep efficiency, the percentage of time in bed actually spent asleep. A young healthy adult may accumulate a small amount of WASO from brief arousals that are never remembered, while WASO rises naturally with age and increases sharply with insomnia, sleep apnea, pain, nocturia, alcohol and some medications.

Clinicians use WASO to characterize sleep-maintenance insomnia, the pattern in which falling asleep is easy but staying asleep is not, and to track whether a treatment reduces nighttime awakenings. It is one of the most common objective endpoints in sleep trials, including trials of cannabinoids, where changes in WASO are reported next to questionnaire scores. Because WASO from actigraphy and from polysomnography are not interchangeable, the measurement method should always be noted when comparing results. Cannabinoids have been studied for sleep maintenance in small trials, and the evidence so far is limited and inconsistent.