Wearable Light Trackers Tested for Circadian Research Accuracy

Light is the dominant environmental cue governing human circadian rhythms, primarily through melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs) with peak sensitivity near 479–490 nm. While laboratory studies have established clear links between light exposure parameters and physiological outcomes, translating these findings to real-world settings demands accurate, wearable light dosimeters capable of capturing spectrally and spatially relevant data continuously.

This study systematically evaluated three commercially available wearable light dosimeters — the LYS Button (RGB, 3-channel), Blue Iris Speck (7-channel, reconstructed SPD from 350–770 nm), and Actiwatch Spectrum Plus (RGB, 3-channel, wrist-worn) — benchmarked against a calibrated Gigahertz S-BTS256 spectroradiometer under multiple lighting conditions including daylight and artificial sources. Evaluation criteria included spectral accuracy, output linearity across illuminance levels, angular (spatial) response, photometric performance, melanopic irradiance calculation accuracy, and thermal stability.

Key findings revealed substantial performance differences among devices. The Blue Iris Speck demonstrated the highest photometric accuracy and most reliable data-logging, making it the strongest candidate for scientifically rigorous circadian light monitoring. Its 7-channel spectral reconstruction enabled more accurate computation of melanopic and α-opic metrics aligned with CIE standards. The LYS Button offered ease of use, lightweight wearability, and smartphone connectivity, but its 3-channel RGB sensor produced lower measurement accuracy — a trade-off relevant for studies prioritizing participant compliance over measurement precision. The Actiwatch Spectrum Plus exhibited the poorest photometric performance among the three, a finding that raises significant caution for researchers drawing causal conclusions about light exposure and biological outcomes from Actiwatch-based data.

The study also highlights the importance of device positioning. Wearing a dosimeter closer to the eye level on a vertical plane best approximates the retinal light dose relevant to circadian biology. The Actiwatch's wrist location introduces systematic underestimation or misrepresentation of actual ocular light exposure. Thermal performance and angular response varied across devices and should be considered when designing multi-day field protocols, particularly in environments with large temperature fluctuations.

From a practical standpoint, the results underscore that circadian metric calculations — including melanopic equivalent daylight illuminance (mel-EDI) and melanopic irradiance (E_mel) per CIE S 026 — depend heavily on the spectral fidelity of the measuring device. Using a spectrally limited or poorly calibrated dosimeter can introduce systematic errors in derived biological metrics, potentially leading to incorrect conclusions in sleep and chronobiology research. The authors recommend that future studies explicitly evaluate and report dosimeter performance characteristics, body-worn position, and calibration status alongside their light exposure data.