Sleep patterns have been linked to weight gain, development of obesity and chronic disease, and mortality in adults. Both shorter and extended sleep have been linked to higher mortality rates in men and women (1, 2). Sleep curtailment has been associated with a higher prevalence of obesity in adults (3, 4), greater weight gain in young adults (5), and development of coronary artery disease and diabetes in women (6-8). Conversely, extended sleep duration (>8 hours) has also been linked to greater risk of coronary artery disease in women (7) and development of type 2 diabetes in men (8). Although lower energy expenditure or underlying disease may explain why longer duration of sleep is associated with obesity, the biologic mechanism for fewer hours of sleep and obesity is more elusive. Persistent sleep deprivation results in physiologic changes, including release of the adipocyte hormones leptin and ghrelin, which stimulate appetite, or other stress hormones such as cortisol, which may lead to overeating and subsequent obesity (9, 10). Major alterations of glucose tolerance occur during sleep, and sleep quality has been shown to markedly affect glucose utilization (11). The first postpartum year is characterized by dramatic alterations in sleep patterns for women (12). Substantial sleep curtailment follows delivery and may persist for several weeks to months, depending on infant size and developmental attributes affecting sleep. Sleep disturbances occur most often within the first month postpartum, particularly after a first birth (12, 13). Few studies have assessed the average maternal sleep time cross-sectionally or longitudinally beyond the puerperium or through the first year postpartum. Clinical studies limited to samples of fewer than 50 women have examined sleep patterns within several weeks after delivery in relation to mood, cognition, and mental health in women (12, 14-16). To our knowledge, the relation of postpartum sleep patterns to other physiologic aspects of maternal health, such as postpartum weight retention, has not been reported. This study examines the association of postpartum sleep duration with substantial postpartum weight retention, defined as a gain of 5 or more kg above preconception body weight at 1 year after delivery. The objectives were to determine the extent to which 1) shorter or longer sleep durations at 6 months postpartum are associated with substantial weight retention at 1 year postpartum, controlling for sociodemographic, prenatal, and behavioral correlates, and 2) change in sleep duration from 6 months to 1 year postpartum is associated with substantial weight retention at 1 year postpartum.