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Summary Obstructive sleep apnea (OSA) causes sleep fragmentation and excessive daytime sleepiness (EDS). OSA has been hypothesised to impair the circadian sleep–wake rhythm, and this dysregulation may in turn exacerbate OSA‐related diurnal symptoms. Hence, this study aimed to assess the sleep–wake rhythm through actigraphy, and its relationship with EDS in patients with untreated OSA. Patients with moderate–severe OSA (apnea–hypopnea index ≥15/h) and healthy controls (HC) underwent a 7‐day actigraphic recording to evaluate the sleep–wake rhythm. Participants underwent a sleep medicine visit and completed the self‐report questionnaires assessing EDS (Epworth sleepiness scale, ESS), sleep quality (Pittsburgh sleep quality index, PSQI), and chronotype (morningness‐eveningness questionnaire, MEQ). This study included 48 OSA patients (72.9% males; mean age 56.48 ± 9.53 years), and 22 HC (45.5% males; mean age 53.73 ± 18.20 years). After controlling for MEQ scores, actigraphic recording showed that the OSA patients present a lower sleep time (p = 0.011) and sleep efficiency (p = 0.013), as well as a higher sleep latency (p = 0.047), and sleep fragmentation (p = 0.029) than the HC. Regarding the sleep‐wake rhythm actigraphic parameters, the OSA patients showed a lower average activity during the most active 10‐hour period (p = 0.036) and a lower day/night activity ratio (p = 0.007) than the HC. Patients with OSA also reported higher ESS (p = 0.005) and PSQI scores (p < 0.001), and a chronotype less of morning type (p = 0.027) than the HC. In conclusion, this study documented a reduced diurnal motor activity and lower day/night activity ratio in OSA patients than in controls. These findings suggest a dysregulation of the circadian sleep–wake rhythm in OSA, possibly related to both EDS and reduced daytime motor activity. [ABSTRACT FROM AUTHOR]