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The relationship between psychological stress and sleep quality is essential for mental health. While self-reported questionnaires are commonly used in epidemiological studies, wearable devices have been increasingly utilized to obtain objective sleep data. This study examined whether daily subjective psychological stress is associated with objectively measured sleep stages in naturalistic (real-world) conditions using wearable devices. We conducted a secondary analysis of a 90-day randomized controlled trial recruited full-time workers with metabolic syndrome (Mets) or high risk of Mets from five Tokyo-based companies. Participants (mean age 43 y; 93% men) used the Fitbit Versa for sleep structure and a dedicated smartphone application for daily questionnaires. Daily psychological stress was assessed using a binary question. Mixed-effects multi-level regression models were used to examine associations between stress and time spent in REM and NREM sleep, respectively. Within-individual psychological stress was positively associated with REM minutes (β: 4.7; 95% CI 2.9, 6.4) and NREM minutes (β: 17.8; 95% CI 13.8, 21.9) in adjusted models. This result suggests that daily individual-level data may be essential for sleep health recommendations. Further research using wearable devices is needed to investigate the possibility of individually optimized interventions targeting psychological stress and its impact on sleep stages.