Actigraphy

Actigraphy is a way of estimating sleep and wake patterns using a small wrist-worn device, called an actigraph, that continuously records movement. A built-in algorithm analyzes periods of activity and stillness to estimate when you were likely asleep and when you were likely awake, over a period that typically spans one to several weeks, far longer than a single overnight sleep study can cover.

The core difference is what's actually being measured. An in-lab sleep study (polysomnography) directly records brain-wave activity, eye movements, and muscle tone, which lets it truly measure sleep stages and definitively distinguish sleep from wakefulness. Actigraphy has no way to record any of that. It infers sleep and wake periods indirectly from movement alone, which makes it a genuinely useful tool for tracking patterns over time, but a fundamentally different kind of measurement than a sleep study provides.

Its main use is evaluating suspected circadian rhythm sleep-wake disorders, conditions in which the body's internal clock is out of step with the schedule a person needs to keep, since actigraphy can objectively document sleep-wake timing over one to two weeks or longer in a way a single overnight study cannot. It's also used to help characterize insomnia patterns over an extended period, often alongside a sleep diary, and sometimes to confirm adequate prior sleep before a multiple sleep latency test.

No. Actigraphy tracks movement, not breathing, so it has no way to detect the airflow reductions or pauses that define obstructive sleep apnea. If sleep apnea is the clinical question, a home sleep apnea test or in-lab sleep study, both of which directly record breathing, is the appropriate test, not actigraphy.

No. Sleep staging (distinguishing light sleep, deep sleep, and REM sleep) requires EEG brain-wave recording, which actigraphy doesn't perform. Actigraphy estimates sleep versus wakefulness as a whole, from movement, but it can't tell you which stage of sleep you were in at a given point in the night.

Circadian rhythm sleep-wake disorders are fundamentally timing problems, so diagnosis depends on documenting when sleep and wake actually happen, night after night, over an extended period. A sleep diary provides a self-reported record of that pattern, and actigraphy adds an objective, movement-based record collected over the same one to two weeks or longer, which is particularly useful when the self-reported pattern is ambiguous or when documentation is needed for a workplace or academic accommodation.

Generally alongside, not instead of. A sleep diary captures your own sense of when you tried to sleep, fell asleep, and woke, while actigraphy adds an objective record from the same period that can support or clarify the diary, especially when the two don't fully agree. Neither is typically considered a full substitute for the other.

Because it estimates sleep from movement rather than measuring it directly, actigraphy can be misled in both directions: lying quietly while awake, for example reading in bed or resting without moving, can be misread as sleep, while restless sleep with frequent movement can be misread as wakefulness. It also can't explain why sleep is disrupted, only when it appears to occur, which is why it's a pattern-tracking tool rather than a diagnostic test for most sleep disorders.

Sometimes. Because adequate prior sleep matters for a reliable MSLT result, some patients are asked to wear an actigraph for a week or more beforehand, so their physician can objectively document their actual sleep pattern going into the test rather than relying on self-report alone.

Most protocols call for continuous wear over one to two weeks, sometimes longer, including through sleep, since the goal is to capture your natural pattern rather than a single unusual night. The device itself is small, similar in form to a fitness tracker or wristwatch, and most people wear it through normal daily activities without significant disruption.