# Wearable Light-and-Motion Datalogers for Sleep/Wake Research: A Review
## 3 Review
### Sensors and Features  
The majority of the dataloggers are equipped with a triaxial accelerometer for motion capture; four have uni- or biaxial accelerometer and two have 6-axis accelerometer (Table 1). Devices with more axes are more sensitive to movements than devices with fewer axes. However, the absolute movement counts do not influence the accuracy of distinction of episodes of greater or lesser activity to a large extent. The in-built software algorithm for assigning these episodes to wake or sleep plays a bigger role. Nevertheless, there is no consensus on algorithms across devices as of now (for more information and references, see for example, [45; 54; 55; 56]). All dataloggers have a built-in light sensor (s) to measure ambient light. This can typically be distinguished by the presence of a small window on the front of the device into which light can enter (Figure 1). Four devices feature a top-half translucent housing to capture more incident light from a wider range of directions (ActiGraph wGT3X-BT, Daqtometer, Daysimeter-D, MSR145). One exception is the Axivity AX3 unit, which has neither the window nor translucent puck casing and is therefore limited to detecting only the changes/trends in ambient light levels, especially when the unit is covered by placement on the inside of the sleeve band offered by the manufacturer. Like the Axivity device, another device, the Daqtometer, provides users with row-unit data only, not converted to specific light values. All other dataloggers provide light values generally comparable to those measured by professional devices. Eleven devices only provide integrated illuminance values (lux) in the photopic range, and six have sensors for specific spectra, including red, green, and/or blue light (R, G, B), measured in irradiance (i.e., W/m\({}^{2}\) or photons/s/m\({}^{2}\)) (Table 1). These devices are useful in studies where it is important to assess the contribution of blue light due to melanopsin-based photoreception (see Section 1). Nine devices measure a wide range of illuminance from \(\leq\)0.2 to 17,000-100,000 lux (Table 1). Dataloggers differ in the type of light sensors they are equipped with. The optical performance characteristics of some dataloggers were investigated and reported by [PERSON] et al., 2017 [12] (Atiwatch 2, Activatch spectrum Plus/PRO, MotionWatch 8, GENEActiv, Daysimeter-D/-S), [PERSON] et al., 2013 [46] (Atiwatch spectrum Plus, Daysimeter-D/-S), [PERSON] et al., 2019 [33] (Kronowise). The within-device variation in the light measurement may also exist, raising the issue of calibration. The published work suggests using the same-ID device on the same subject throughout the study [12; 46; 57]. In addition to light and motion sensors, ten dataloggers have other sensors, such as temperature sensors (nine devices). ActTrust/ActTrust 2 and Kronowise can measure wrist temperature, for which the thermistor is located as close as possible to the inner surface of the device in contact with the skin. In other cases, the thermistor is located centrally or close to the front of the device, recording neither a true skin nor a true ambient temperature (denoted as \"External temperature\" in Table 1). In addition to the external temperature, four devices (LightWatcher, MSR145, MMC and MMS) can measure other environmental parameters (Table 1). ActiGraph wGT3X-BT can record heart rate via Bluetooth connection with a Polar chest belt monitor. Three devices--Atiwatch spectrum Plus/PRO, Micro Motionlogger Watch, and ActiGraph wGT3X-BT--have a wear time sensor (capacitive touch technology) that can help distinguish, for example, whether the subject has been sleeping during daytime or has just removed the device, which is important in research. ActiGraph wGT3X-BT, Kronowise and Axivity (AX6 version) can additionally determine the position of the body using a gyroscopic inclinometer sensor. Usually, researchers can program which sensory channel is enabled for recording, but some dataloggers do not have this option (not specified in Table 1).