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Summary Sleep trackers are used widely by patients with sleep complaints, however their metrological validation is often poor and relies on healthy subjects. We assessed the metrological validity of two commercially available sleep trackers (Withings Activité/Fitbit Alta HR) through a prospective observational monocentric study, in adult patients referred for polysomnography (PSG). We compared the total sleep time (TST), REM time, REM latency, nonREM1 + 2 time, nonREM3 time, and wake after sleep onset (WASO). We report absolute and relative errors, Bland–Altman representations, and a contingency table of times spent in sleep stages with respect to PSG. Sixty‐five patients were included (final sample size 58 for Withings and 52 for Fitbit). Both devices gave a relatively accurate sleep start time with a median absolute error of 5 (IQR −43; 27) min for Withings and −2.0 (−12.5; 4.2) min for Fitbit but both overestimated TST. Withings tended to underestimate WASO with a median absolute error of −25.0 (−61.5; −8.5) min, while Fitbit tended to overestimate it (median absolute error 10 (−18; 43) min. Withings underestimated light sleep and overestimated deep sleep, while Fitbit overestimated light and REM sleep and underestimated deep sleep. The overall kappas for concordance of each epoch between PSG and devices were low: 0.12 (95%CI 0.117–0.121) for Withings and VPSG indications 0.07 (95%CI 0.067–0.071) for Fitbit, as well as kappas for each VPSG indication 0.07 (95%CI 0.067–0.071). Thus, commercially available sleep trackers are not reliable for sleep architecture in patients with sleep complaints/pathologies and should not replace actigraphy and/or PSG. [ABSTRACT FROM AUTHOR]