Wake After Sleep Onset as a Digital Biomarker

Wake after sleep onset, universally abbreviated to WASO, is the total time a person spends awake between first falling asleep and their final awakening. It is the measure that captures sleep maintenance as opposed to sleep initiation, and it is the primary complaint of a large share of people with insomnia, who fall asleep without difficulty but cannot stay asleep. WASO is also one of the outcomes recommended by consensus for insomnia trials, alongside sleep onset latency and sleep efficiency, precisely because it isolates a distinct and treatable problem. As a digital biomarker it is estimated nightly at home. That is valuable because sleep maintenance varies substantially from night to night, and a laboratory study captures one or two nights while a wearable captures a hundred.

Actigraphy infers wake from movement: epochs with activity above a threshold within the sleep period are classified as wake, and the total is summed. Devices with photoplethysmography add heart rate and heart rate variability, which help identify arousals that produce little movement. The measure is bounded by two events the device must also estimate, sleep onset and final awakening, so errors in either shift WASO directly. The reference standard is polysomnography, which scores wake from the electroencephalogram regardless of whether the sleeper moved. This is where the fundamental difficulty lies: a person lying still and awake in the dark generates almost no accelerometer signal, so movement-based methods have consistently lower specificity for wake than sensitivity for sleep. Research protocols average across at least seven nights.

WASO is a standard endpoint in insomnia pharmacotherapy and cognitive behavioural therapy trials, where treatment aims specifically to consolidate fragmented sleep. It is used as a monitoring measure in depression, chronic pain, menopause and neurodegenerative disease research, where night-time awakenings track symptom burden and often predict next-day function better than total sleep time does. In the public DiMe endpoint library WASO is registered across trials in sleep, mental health and urogenital conditions, including as a primary endpoint. Because it is the measure wearables estimate least reliably, well-designed trials either validate the device against polysomnography in a subsample or treat WASO as a supporting rather than a primary outcome, and pair it with the Insomnia Severity Index.

This is the sleep measure wearables get least right. Because quiet wakefulness produces almost no movement, movement-based methods systematically under-detect wake, and specificity for wake in validation studies is frequently well below fifty percent. The bias runs in one direction, so WASO is typically underestimated, and the underestimation is largest in people with insomnia, meaning the error is worst in the population the measure is designed to serve. WASO also inherits any error in the sleep onset and final awakening boundaries. Absolute values should not be compared across device brands, and treatment effects are more trustworthy than absolute levels.

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