Background: The popularity of wearable health monitoring devices, such as Fitbits, is on the rise. However, their applicability in various research demographics remains uncertain, particularly among postpartum women facing significant sleep challenges. Objective: This study aimed to evaluate the consistency between Fitbit Charge 6 and structured interview questions in measuring sleep quality among breastfeeding mothers. Methods: A cross-sectional study utilizing random convenience sampling was conducted in Ho Chi Minh City, Vietnam, involving 208 breastfeeding women one month postpartum. The participants used the Fitbit Charge 6 device continuously for four days and three nights to gather daily sleep information; then, they were administered a structured sleep questionnaire via home interviews. Results: The correlation between the total sleep time (TST) recorded by both the device and interview was weak (r = 0.28), as was the correlation for short nap duration (r = 0.25). The agreement between the Fitbit Sleep Score and sleep satisfaction from the questionnaire was 79.9%, with a notably low Kappa index (0.02), indicating a lack of strong agreement. Conclusion: The Fitbit Charge 6 did not show reliable correlation with the subjective sleep data reported by one-month postpartum breastfeeding women. Further research employing rigorous comparative studies is necessary to assess the validity and reliability of sleep data collection methods for this demographic.