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This longitudinal study was designed to examine objective sleep parameters of older primiparous Japanese women during the first 4 months postpartum using actigraphy. The participants were 18 older primiparae (Mean (SD) = 37.06 (2.62) years, range 35–44 years) who gave birth to healthy neonates at one of three urban Japanese hospitals. Objective sleep quality was measured using actigraphy for 48 h at 1, 2 and 4 months postpartum. The Friedman test was used to test for differences in sleep parameters across time. Sleep duration (SMIN) increased significantly from 2 months (Mean (M) = 301.94 min) to 4 months (M = 372.78 min). Sleep efficiency (SE) increased significantly from 1 month (73.52%) to 2 (86.66%) and 4 months (89.05%). Waking after sleep onset (WASO) decreased significantly from 1 month (M = 114.64 min) to 2 (M = 40.18 min) and 4 months (M = 38.36 min) and long waking episodes (LWEP) significantly decreased from 1 month (4.67) to 2 (2.69) and 4 months (3.12). Persistent postpartum sleep problems can be a sign of postpartum depression as well as health problems among infants.