# Wearable Light-and-Motion Datalogers for Sleep/Wake Research: A Review
## 3 Review
### Battery  
The battery is a critical aspect for reliable performance, related to the problems of temporal unavailability of the device (no charged battery at the moment) or accidentally missing data (due to energy depletion during the recording). Two kinds of batteries of coin-type are used in the devices--rechargeable (lithium-ion polymer battery, nine devices) and non-rechargeable (eight devices). The former are not intended to be replaced by the user, while the latter are user-replaceable (except Daysimeter-D) (Table 1).  
It is usually battery life and not device memory that limits recording time. Although recording time is commonly declared to be as long as 21-90 days (only in four cases is the duration less; Table 1), 7-14 days is considered a safe period. A warning from the device on low battery is a useful important feature. The devices vary greatly with respect to this feature: no warning, stop flashing of the work status indicator, audible warning right after the recording stopped, flashing or audible warning somewhat before the recording will stop, constantly working charge level indicator (Table 1). Only the latter two features provide sufficient time for the datalogger wearer and researcher to plan an extraordinary meeting for the battery (or device) replacement to prevent impending data loss.  
Figure 1: Reviewed dataloggers (pictures taken from manufacturers’ websites or scientific articles (mentioned in Table S1)).  
Current battery charge (in volts or % of the maximal voltage) and sometimes the predicted runtime (e.g., in MSR145) can be checked with the supplied software. The check can be done at any time during the data acquisition, as this does not normally stop the recording. Unfortunately, the voltage does not basically reflect the energy capacity of the battery, so the actual operating time may appear to be significantly less than estimated. Generally, the runtime of a rechargeable battery is more difficult to predict than a non-rechargeable battery. For a rechargeable battery, it depends on how many charge/re-charge cycles the battery underwent previously, whether it has been well maintained when the device was not in run mode (a full charge is recommended after each use and every 2-6 months thereafter, as zero-energy battery live short), and the expected overall battery life (when kept unused from the beginning).  
A disadvantage of the rechargeable battery is that the recharge usually takes \(\sim\)3 h, and if there is no other pre-charged datalogger to immediately continue the recording in a subject who came to an interim visit to the investigator, this can turn into a waiting time inconvenient for both persons. A further disadvantage is a need to send the device to the manufacturer to replace the battery after every 2-3 years of use. One more disadvantage concerns wearables, which require a docking station for charging (three devices, Table 1)--any intermediate technical element increases the risk of arising an additional problem, such as poor contact of the datalogger to a docking station or unavailability of the dock at the right time.  
The situations in which a researcher should suspect a faulty battery also include a very short charging time (for rechargeable battery) and unreliable light or motion levels appearing gradually on the analyzed actogram. A delayed start at a defined time helps to save battery power, and many devices are provided by this software mode (Table 1). Optimal operating temperature ranges between 5-40 \({}^{\circ}\)C; lower or higher temperatures will shorten the battery life and reduce the battery capacity, especially outside the range \(-20\div 60\)\({}^{\circ}\)C. Nevertheless, even when the battery is exhausted, the recorded data remains in the unit's memory--they are non-volatile.