Actigraphy in sleep trackers

The timing and regularity of a person's physical activity and sleep affect metabolism and the risk of developing obesity, type 2 diabetes and several other diseases. These days, many people suffer from a variety of sleep disorders. One way of non-invasive objective assessment of activity rhythmicity and habitual sleep-wake rhythm over several days is the use of actigraphy.

What is actigraphy?

Actigraphy is a non-invasive method of monitoring or assessing a person's physical activity using an actigraph, a compact device that assesses a person's motor activity using data from a 3-axis accelerometer. For this purpose, algorithms are used to process the acquired data, classifying the state as sleep or wakefulness, which allows us to estimate the time in bed, the duration of falling asleep and night awakenings, the duration of sleep, its efficiency, and the stability of the sleep-wake cycle.

An actigraph is a wristwatch-like device that assesses a person's motor activity during a night's rest. Such data can be presented in graphical form showing activity over time or in quantitative form. Despite the complexity of the algorithms used to analyze the indicators, they work on a general principle: subject movement indicates wakefulness, and lack of movement indicates sleep. The actigraph records data every minute, which allows for the calculation of various sleep continuity parameters.

To determine the average sleep duration, it is sufficient to record data for 3 days, including one day off. To assess the sleep-wake rhythm, recordings should be made over a 7-day period, including weekends, when waking times are usually unrestricted. If an unstable sleep-wake rhythm needs to be studied, such as with insomnia or shift work, a longer follow-up period of up to 2 weeks may be required.

What is the usefulness of actigraphy?

This method has the following capabilities:

Allows for objective assessment of sleep continuity, eliminating the influence of self-assessment or observer bias.
Does not require strict adherence from the patient.
It has high temporal resolution, allowing you to record short periods of wakefulness of up to 30 seconds.
Allows continuous monitoring for up to 60 days.
In combination with data from a sleep diary, it helps to identify discrepancies between objective measures of sleep and subjective perception (a situation where patients appear to be asleep according to objective measures but do not consider themselves to be so).
Promotes more accurate recording of sleep continuity in the diary by the patient.
Allows monitoring of adherence to therapy recommendations, such as restrictions on short daytime naps and adherence to established sleep and wake schedules.

Actigraphy is also used in sleep studies. In this way, research that would otherwise fail due to cost becomes possible. One example was a comparative study of various sleep parameters compared to sleeping in a hammock versus sleeping in bed.

Indications for actigraphy

Actigraphy is a cost-effective method for diagnosing certain sleep disorders such as insomnia, circadian sleep-wake disturbances, and increased sleepiness (hypersomnia).

During the initial assessment phase, to determine adherence to a prescribed treatment regimen or to evaluate changes in therapy, actigraphy may be used to:

to identify circadian rhythm irregularities (e.g. phase shifts, free-flowing rhythms, etc.);
Identification of patients with significant misperception of their sleep state (so-called paradoxical insomnia).

In the treatment phase, actigraphy can be used for:

objective assessment of achievements in sleep continuity and the degree of patient adherence to the therapy regimen (especially in people with dementia, aphasia, mental disorders, from whom it is impossible to obtain reliable information during the interview);

To document patient adherence to the prescribed delayed bedtime phase with a prohibition on sleeping during the day and instructions to get out of bed upon awakening at night (stimulus control) and at the prescribed time in the morning.

How Actigraphy Differs from Other Sleep Assessment Tools

While actigraphy is a powerful and convenient tool, it’s important to understand how it compares to other methods, particularly polysomnography (PSG)—the gold standard for diagnosing sleep disorders. Both have distinct strengths and limitations that cater to different needs.

Feature	Actigraphy	Polysomnography (PSG)
Cost	Affordable. Suitable for routine monitoring.	Expensive. Often requires insurance or out-of-pocket payment.
Convenience	Wearable, non-invasive. Tracks sleep in natural environments.	Requires an overnight lab stay in a controlled environment.
Data Collected	Movement-based sleep patterns (sleep/wake states).	Detailed physiological signals (brain waves, breathing, muscle activity).
Accuracy	Moderate. Ideal for detecting general patterns.	High. Provides in-depth diagnostics for sleep disorders.
Use Case	Long-term, everyday monitoring of sleep rhythms.	Short-term diagnostic studies for complex sleep issues.
Monitoring Duration	Can track sleep continuously for weeks or months.	Usually limited to one or two nights in a clinical setting.

When to Choose Actigraphy

Actigraphy is a great option for:

Everyday Sleep Monitoring: Provides insights into real-life sleep behaviors without disrupting routines.
Cost-Conscious Users: A more budget-friendly option compared to expensive lab-based studies.
Long-Term Analysis: Offers continuous tracking over days or weeks, which is critical for conditions like circadian rhythm disorders or insomnia caused by lifestyle factors.
Pediatric or Special Needs Populations: Ideal for children or individuals who may not tolerate the intrusive setup of PSG.

Actigraphy in Modern Wearable Technology

The integration of actigraphy into consumer-grade wearables has revolutionized sleep tracking. Devices like smart rings, fitness trackers, and advanced sleep monitors use actigraphy to deliver actionable insights directly to the user’s smartphone.

How Wearables Use Actigraphy

1. Tracking Sleep Stages
Many devices can distinguish between light, deep, and REM sleep stages, providing a detailed picture of the sleep cycle. This feature is particularly useful for identifying patterns such as insufficient deep sleep or frequent REM disruptions.
2. Activity Metrics
Measure daily movement, helping users track their active hours versus sedentary time. Provide insights into how daytime activity levels affect sleep quality.
3. Health Recommendations
Wearables use actigraphy data to generate personalized tips, such as improving bedtime routines, adjusting wake-up times, or increasing daytime activity for better sleep.

Advanced Integration with Other Metrics

Modern wearables combine actigraphy with additional health sensors, including:

Heart Rate Variability (HRV): Used to detect stress levels and recovery.
SpO2 (Blood Oxygen Saturation): Helps identify potential breathing irregularities during sleep.
Body Temperature: Tracks subtle temperature fluctuations that can indicate stress, illness, or changes in sleep quality.

Popular Devices Utilizing Actigraphy

RingConn: Known for its sleep staging, readiness scores, and holistic health tracking.
Fitbit Sense & Charge Models: Offer sleep insights combined with fitness tracking.
Whoop Band: Focuses on recovery metrics, including sleep and strain levels.

By blending actigraphy with other data points, these devices empower users to better understand their sleep health, identify areas for improvement, and make lifestyle adjustments.

Challenges and Limitations of Actigraphy

Although actigraphy is an invaluable tool for sleep and activity monitoring, it does have some limitations:

1. Misclassification of Motionless Wakefulness
Actigraphy primarily measures movement, which means periods of still wakefulness (e.g., lying in bed awake but not moving) can be misclassified as sleep. This can lead to slight overestimations of total sleep time.
2. Reliance on Algorithms
The accuracy of actigraphy depends heavily on the algorithms used by the device. Different brands may use proprietary methods to interpret movement data, which can result in varying levels of precision. For example, cheaper devices may provide less reliable insights compared to premium models like the Oura Ring and RingConn.
3. Need for Supplementary Data
Actigraphy works best when paired with additional data points to provide context, such as:

Sleep Diaries: Help correlate objective data with subjective perceptions.
Heart Rate Monitoring: Enhances accuracy by detecting periods of restlessness or elevated heart rate.
Lifestyle Factors: Daytime activity, stress levels, and diet can all impact sleep but may not be fully captured by actigraphy alone.

4. Limited Diagnostic Capabilities
While actigraphy is excellent for general sleep monitoring, it lacks the depth needed to diagnose complex sleep disorders such as sleep apnea. For such conditions, polysomnography remains the preferred method.

Why These Challenges Don’t Undermine Actigraphy

Despite these limitations, actigraphy remains one of the most practical tools for everyday sleep monitoring. It offers a unique combination of accessibility, affordability, and long-term tracking capabilities that make it indispensable for:

Identifying trends over time.
Gaining insights into natural sleep behaviors.
Monitoring progress during sleep therapy.

By integrating actigraphy into wearable devices, users can take a proactive role in improving their sleep health while keeping costs low and convenience high.

Conclusion

Actigraphy is an easy-to-use method that allows the assessment of sleep-wake rhythm in different groups of patients. This method is recommended for assessing sleep characteristics when polysomnography is not available, as well as for diagnosing circadian sleep-wake rhythm disorders, insomnia, and for assessing sleep and wakefulness in the daily monitoring of other physiologic parameters.

FAQs About Actigraphy

1. What is the difference between actigraphy and polysomnography?
Actigraphy tracks movement to estimate sleep, while polysomnography measures brain activity, breathing, and other physiological signals for detailed diagnostics.
2. Can smart rings provide actigraphy data?
Yes, devices like the Oura Ring and RingConn use actigraphy sensors to track sleep and activity patterns.
3. How long should I track sleep with actigraphy?
3-7 days is sufficient for general insights, while 2 weeks may be required for unstable sleep patterns like those caused by shift work.
4. What are the limitations of actigraphy?
It may misclassify still wakefulness as sleep and requires supplementary data for greater accuracy.
5. Is actigraphy suitable for children or older adults?
Yes, actigraphy is effective for various age groups, including those unable to communicate sleep issues effectively.