Actigraphy is a method of tracking your body’s movement over days or weeks using a small, wearable sensor, typically on the wrist. The device records how much and how often you move, then uses algorithms to estimate when you were asleep and when you were awake. It’s widely used in sleep medicine to diagnose insomnia, circadian rhythm disorders, and other conditions where understanding real-world sleep patterns matters more than a single night in a lab.
How the Device Works
An actigraph contains a tiny accelerometer built with microelectromechanical systems (MEMS) technology. Inside the sensor, two plates sit close together. One is fixed, and the other shifts when your body moves. That shifting changes the electrical charge between the plates, and the device converts that change into a digital signal, sampling your movement 30 times per second. A digital filter then isolates the frequency range associated with human body movement (roughly 0.25 to 2.5 Hz), filtering out vibrations from vehicles, machinery, or other non-body sources.
Older devices measured movement in only one direction (up and down), but current models track three axes of motion: vertical, front-to-back, and side-to-side. The device combines these into a single composite score that captures the full picture of your physical activity. All of this data is stored on the device and downloaded later for analysis.
What It Measures
Raw movement data isn’t useful on its own. Software applies validated algorithms to convert motion counts into sleep and wake estimates. The key metrics generated include:
- Total sleep time: how many minutes you actually slept during the night
- Sleep efficiency: the percentage of time in bed that you spent asleep
- Wake after sleep onset: how many minutes you were awake after initially falling asleep
- Number of awakenings: how many times you woke up during the night
- Sleep onset latency: how long it took you to fall asleep
Together, these metrics paint a detailed picture of your sleep architecture over multiple nights. Because actigraphy captures data continuously across days or weeks, it reveals patterns that a single overnight sleep study in a clinic can’t.
How Accurate Is It?
The gold standard for measuring sleep is polysomnography (PSG), which uses electrodes attached to your scalp, face, and body to monitor brain waves, eye movements, and muscle activity in a sleep lab. Actigraphy can’t match that level of detail, but it performs reasonably well for estimating sleep.
Compared to polysomnography, actigraphs show overall accuracy of 81% to 86% for distinguishing sleep from wakefulness. Total sleep time estimates correlate fairly well with lab measurements. The weak spot is detecting wake after sleep onset, where agreement with polysomnography is poor. This happens because actigraphy relies on movement: if you’re lying still but awake, the device assumes you’re asleep. Studies have found that roughly 11% to 14% of wake periods get misclassified as sleep, particularly during long stretches of quiet wakefulness like the kind people with insomnia experience.
Two commonly used scoring algorithms handle this tradeoff differently. One (the Cole-Kripke algorithm) is better at detecting sleep, with sensitivity between 88% and 96%, but catches only 35% to 64% of wake periods. The other (the Sadeh algorithm) is slightly less sensitive to sleep but better at identifying wakefulness. Clinicians choose between them based on the condition being evaluated.
What Conditions It’s Used For
The American Academy of Sleep Medicine (AASM) has issued clinical guidelines recommending actigraphy for several specific situations, in both adults and children.
For insomnia, actigraphy serves as a useful companion to sleep diaries. While self-reported sleep logs remain the primary tool for diagnosing insomnia, people often misjudge how long they slept or how many times they woke up. Actigraphy provides an objective counterpoint. It’s particularly valuable for tracking treatment response, comparing sleep quality before and after an intervention. The combination of total sleep time, sleep onset latency, and number of awakenings gives clinicians a way to characterize whether someone has trouble falling asleep, staying asleep, or waking too early.
For circadian rhythm disorders, where the body’s internal clock is misaligned with the desired sleep schedule, actigraphy is especially powerful. Conditions like delayed sleep-wake phase disorder (common in teenagers who can’t fall asleep until 2 or 3 a.m.) or shift work disorder require tracking sleep patterns across many 24-hour cycles. A single night in a sleep lab can’t reveal these patterns, but two weeks of actigraphy data can.
Actigraphy also plays a role in diagnosing excessive daytime sleepiness. Before a patient undergoes a Multiple Sleep Latency Test, which measures how quickly they fall asleep during the day, clinicians need to confirm the patient has been getting enough sleep in the preceding weeks. Actigraphy documents this objectively. It’s also used alongside home sleep apnea testing devices to estimate how long a patient actually slept during the recording, which improves the accuracy of those tests.
One notable limitation: the AASM strongly recommends against using actigraphy to diagnose periodic limb movement disorder, a condition involving repetitive leg movements during sleep. That diagnosis requires electromyography, which directly measures muscle electrical activity, something an accelerometer can’t replicate.
What Wearing One Looks Like
An actigraph is typically a small, watch-like device worn on the non-dominant wrist. It’s lightweight and waterproof enough for continuous wear. Patients are usually asked to wear it for seven consecutive days, though research suggests that four days with at least eight hours of wear time per day is the minimum needed for reliable estimates. During the monitoring period, you go about your normal routine. There are no wires, no electrodes, and no need to sleep in a clinic.
Most clinicians ask you to keep a simple sleep diary alongside the device, noting when you went to bed, when you got up, and any times you removed the device. This helps the software distinguish between periods when you took the device off and periods when you were simply lying still.
Clinical Actigraphs vs. Consumer Wearables
If you own a smartwatch or fitness tracker, you might wonder whether it does the same thing. There’s overlap, but important differences exist. Clinical actigraphs like the ActiGraph GT9X are FDA-cleared, validated against polysomnography in peer-reviewed studies, and equipped with research-grade sensors including gyroscopes and magnetometers alongside the accelerometer. Consumer devices from companies like Apple, Fitbit, and Oura also contain accelerometers and add extras like heart rate monitors and blood oxygen sensors, but their algorithms for converting raw data into sleep estimates are proprietary and undisclosed.
This lack of transparency is a core challenge. Because manufacturers don’t share how their devices calculate sleep metrics, researchers can’t fully evaluate or compare them. Data from different consumer devices may not be equivalent, even when measuring the same person on the same night. Wear location also matters: clinical actigraphs are sometimes worn on the hip for activity research, while consumer devices sit on the wrist or finger, and these positions capture movement differently.
Consumer wearables can give you a general sense of your sleep trends, but they aren’t interchangeable with medical-grade actigraphy for clinical decision-making.
Known Limitations
Actigraphy’s fundamental limitation is that it infers sleep from the absence of movement. Any time you’re motionless but awake, the device is likely to score that period as sleep. This is most problematic for people with insomnia, who may spend long stretches lying in bed unable to sleep. It also means the device can overestimate total sleep time and underestimate wakefulness.
The reverse problem occurs too. If the device is set down on a surface near a source of vibration, like a nightstand next to a fan, it may register small motion artifacts and classify a non-wear period as wakefulness. Algorithms attempt to handle this by flagging periods of 90 minutes or more with zero activity as likely non-wear, but edge cases remain.
Despite these limitations, actigraphy fills a gap that no other tool covers as well. Polysomnography is expensive, disruptive, and captures only one or two nights. Sleep diaries depend on memory and subjective perception. Actigraphy offers weeks of objective, real-world data at relatively low cost, which is why it has become a standard part of sleep medicine evaluation.