The role of actigraphy in sleep medicine

Actigraphy has been used for more than 60 years to objectively measure sleep–wake rhythms. Improved modern devices are increasingly employed to diagnose sleep medicine disorders in the clinical setting. Although less accurate than polysomnography, the chief advantage of actigraphs lies in the cost-effective collection of objective data over prolonged periods of time under everyday conditions. Since the cost of wrist actigraphy is not currently reimbursed, this method has not enjoyed wide acceptance to date. The present article provides an overview of the main clinical applications of actigraphy, including the recommendations of specialist societies.


The role of actigraphy in sleep medicine
Behavior in the home setting plays an important role in many sleep-related disorders. Despite the availability of actigraphy, longitudinal disease and treatment courses are usually recorded using subjective reports (e.g., sleep logs/sleep diaries).
Actigraphy has been used for more than 60 years to objectively measure sleep-wake rhythms [1]. The procedure makes it possible to measure and evaluate movement and other parameters such as light exposure over a prolonged period of time.
In recent years, actigraphy has been increasingly used in the clinical setting. Modern medical actigraphs are more accurate and reliable due their improved piezoelectric motion sensors, lithium batteries, and enhanced storage capacities.
Current devices are able to record motorbehavioroverperiods ofup tomonths. Their improved waterproofing and low weight make them suitable for prolonged use under natural conditions, even on moving surfaces (e.g., on ships) [2]. Thus, insight can be gained into the motor phenomena of activity and rest phases as well as of circadian rhythms.
Since the cost of wrist actigraphy is not reimbursed, this method has not enjoyed wide acceptance to date. The present article provides an overview of its main clinical applications (. Table 1).
If one puts the main measuring instruments used in sleep medicine in order of accuracy, modern actigraphs rank below the accuracy of polysomnography for the majority of variables measured. Their chief advantage lies in the cost-effective collection of objective data over prolonged periods of time, usually 7-14 days, under everyday conditions. Particularly when investigating insomnia, hypersomnia, and circadian rhythm disorders, longer measurement periods significantly improve validity [4]. For a correct assessment of the sleep onset spectrum, it is important to have a detailed knowledge of the different assessment procedures (. Fig. 1). This plays a special role, for example, in the assessment of insomnia patients, as there can be considerable discrepancies between sleep protocols and actigraphy findings.

Technical features
Newer actigraphs record motion in up to three axes. The recorded data are usually processed in a frequency range of 0.25-3 Hz with band-pass filters before they are saved. As a general rule, the epochs used in actigraphy devices can be freely selected; in sleep medicine practice, they are usually 30 or 60 s.
For adaptation of the scoring algorithms, empirical values or laboratory   are as yet no standardized scoring recommendations negatively affects the objectivity of scoring and inter-rater reliability [6].

Data acquisition and cleansing
Wrist actigraphy usually measures daynight rhythms in daily life over 1-4 weeks [7]. The ideally waterproof devices are worn on the non-dominant hand for 24 h. Additional information such as subjective total sleep time and quality can be recorded in a sleep diary. This can be used when discussing findings with the patient, in order to reconstruct everyday situations together with the patient and correlate these with the recorded actigraphy [8].
The use of actigraphy event markers as soon as a sleep attempt begins (eye closure, corresponding to actigraphy lightout) and ends (opening eyes in the morning, getting out of bed, corresponding to actigraphy light-on) has proven successful.
At the same time, sleep opportunity, which is often spent on other activities (e.g., watching TV, reading, eating)canbe distinguished from actual sleep attempts. Nighttime sleep interruptions, such as visits to the toilet, are not marked by patients.
Thus, the actigraphy system is able to record the length of sleep, which is needed later to calculate the actigraphy variables (see . Table 2; [9]).
After downloading the actigraphy data, manual data cleansing should be performed. This makes it possible to significantly improve the actigraphy report [10]. Sleep-related data are then calculated, and an actogram generated with visualization of the day-night rhythm (see . Fig. 2). In clinical practice, automated analysis is generally used to this end [11,12].
The graphic representation of the actogram (see example in .

Figs. 2 and 3)
can be readily used in the consultation process and compared with the patient's subjective perception.

Actogram interpretation
An example actogram of a care home resident recorded over a 1-week period is presented in . Fig. 2.

Actigraphy in social jetlag phenomenon
An extract from a 3-week actigraphy of a patient with social jetlag (SJL) is presented in . Fig. 3.

Actigraphy vs. wearables in sleep medicine practice
In sleep medicine consultations, patients often present measurements that have been recorded using smartphones or commercially available wearables.
A number of points need to be considered when interpreting these data: 1. These devices are often unvalidated compared to standard sleep medicine The role of actigraphy in sleep medicine Abstract Actigraphy has been used for more than 60 years to objectively measure sleep-wake rhythms. Improved modern devices are increasingly employed to diagnose sleep medicine disorders in the clinical setting. Although less accurate than polysomnography, the chief advantage of actigraphs lies in the cost-effective collection of objective data over prolonged periods of time under everyday conditions. Since the cost of wrist actigraphy is not currently reimbursed, this method has not enjoyed wide acceptance to date. The present article provides an overview of the main clinical applications of actigraphy, including the recommendations of specialist societies.

Keywords
Circadian rhythm · Sleep-disordered breathing · Insurance, health · Polysomnography · Sleep initiation and maintenance disorders Thus, medical actigraphy devices continue to be recommended for clinical use [18]. Due to the widespread use of wearables and smartphone apps, their validation will remain an important challenge in the coming years for medical societies, some of which have set up their own task forces [19][20][21].

Review
Motor activity

ICSD-3 recommendations on the use of actigraphy
The current version of the International Classification of Sleep Disorders (ICSD-3)recommendsactigraphyasadiagnostic tool to supplement classic sleep questionnaires and sleep diaries. Particularly in circadian rhythm disorders does actigraphy have an important role (. Table 4).

Recommendations on clinical use of actigraphy (AASM task force)
In 1995 and 2002, the American Academy of Sleep Medicine (AASM) classified actigraphy as a suitable research instrument but deemed its clinical benefit to be unclear. Manual evaluation of actigraphy data was recommended [23,24]. In 2018, based on current evidence, the AASM issued new recommendations on the use of actigraphy in clinical routine (. Table 5; [25]).

Reimbursement aspects
In Germany, actigraphy has not as yet been included in the catalog of services covered by health insurers. To date, it has been billable to the patient as an individual health service (IGeL). A revised catalog of services for privately insured patients includes actigraphy for at least 7 days as a reimbursable service. A policy decision in this regard is pending.
In Switzerland, actigraphy is included in the catalog of mandatory health insurance services, assuming the investigation is carried out at a sleep center recognized by the Swiss Society for Sleep Research, Sleep Medicine, and Chronobiology (SGSSC). If this is not the case, the costs need to be cleared with the health insurance's medical officer before the investigation is performed.
No provision is made in primary care for the reimbursement of actigraphy in Austria at present.

Discussion
In healthy populations, actigraphy is able to reliably and adequately record total sleep time and sleep onset time within a 24-h period. Particularly in the case of longitudinal measurements, these results can be placed in a wider context: variances in sleep duration and time of sleep onset; weekly structure (SJL); diagnosis of non-24 syndromes by means of period analysis; estimation of chronotype (relation between time of sleep onset and natural light-dark change [photoperiod] and extent of SJL).
In recent years, actigraphy has been increasingly crystallizing as a clinical tool to diagnose disorders in sleep medicine [26]. In combination with a sleep diary, important information on sleep behavior in the home setting can be obtained over periods of 1-4 weeks with good cost efficiency.
When combined with detailed patient history taking, actigraphy is able to estimate the main sleep-related variables: total sleep time (TST), sleep efficiency (SE), and wake time after sleep onset (WASO) can be determined in numerous sleep disorders.
There are limitations to the use of actigraphy alone for the sleep assessment [27]. It underestimates TST in patients with severe daytime sleepiness, in patients with lower sleep fragmentation, and in patients with more severe sleepdisordered breathing. Actigraphy overestimates TST in patients with high sleep fragmentation, milder severity of sleepdisordered breathing, and short sleepers.
Thus, e.g., motionless wakefulness, as is more common in insomnia patients, is challenging to identify with this method. In order to record the high variability in sleeping patterns in insomnia patients, longer recording periods should be chosen [4].
Actigraphy generally shows good concordance with subjective patient reports on day-night rhythms (e.g., sleep-onset and sleep-offset times). However, there can be significant discrepancy between sleep quality variables such as WASO or SE and the patient's subjective perception. This mismatch needs to be taken intoaccountatthe treatmentdesign stage, in order to be able to develop sustainable treatment plans together with the patients.
The automated evaluation algorithms currently used could be further refined in the future once data collection processes have been adapted to additionally use artificial intelligence [28].
In the authors' opinion, the factors that remain essential for the successful application of actigraphy in sleep medicine include the clarification of reimbursement  If available, actigraphy for at least 7, but ideally 14 days including work/ school days and weekends/leisure time K the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.