Many young children experience insufficient or poor quality sleep, which may have implications for adjustment and cognitive performance. This study tested group-level changes and rank-order stability in both daytime and nighttime sleep parameters over a six-month period, from fall to spring, among children receiving high-quality, center-based daycare. A total of 68 preschoolers (54% girls; M(subscript age) = 3.80 years, SD = 0.68) participated. Sleep was assessed via actigraphy for seven days and nights; sleep duration (actual sleep minutes) and quality parameters were derived. Analyses of group-level changes indicated that children's daytime and nighttime sleep duration did not change significantly from fall to spring. Nighttime sleep quality showed significant improvement, however, such that children had higher sleep efficiency in the spring than in the fall. Rank-order stability in both nighttime and daytime measures of sleep duration and quality was moderate, and stability in daytime sleep quality was low. Results add to a sparse literature examining stability in sleep parameters in young children using actigraphy.