[sheefrex]

They're not as accurate as polysomnography but there is some correlation which should allow these measurements to serve as a useful proxy over 100-180 days. See this review for example:

"The findings of this systematic literature review indicate that the devices with higher relative agreement and sensitivities to multistate sleep (ie, Fitbit Charge 4 and WHOOP) seem appropriate for deriving suitable estimates of sleep parameters. However, analyses regarding the multistate categorization of sleep indicate that all devices can benefit from further improvement in the assessment of specific sleep stages."

Source: https://mhealth.jmir.org/2024/1/e52192/PDF