Abstract Introduction Short sleep duration and poor sleep quality have each been associated with obesity and weight gain. However, less is known regarding how sleep may impact attempted weight loss. The purpose of this study was to investigate the associations between sleep duration and sleep quality, both independently and in combination, with weight loss in a 12-month behavioral weight loss intervention. Methods Young to middle-aged adults who were overweight or obese (N=296) completed a 12-month behavioral weight loss intervention, with weight assessed at baseline, 6 and 12 months. Sleep duration and quality were derived from the Pittsburgh Sleep Quality Index. Analyses examined the change in sleep over time and the association between baseline sleep and changes in sleep with 6- and 12-month weight loss following adjustment for relevant covariates including age, gender, race, education, baseline body mass index, and baseline risk for sleep apnea. Results Participants (with an average baseline weight of 97.0±1.0 kg) lost 6.6±1.1 kg (6.8%) and 6.7±1.2 kg (6.9%) at 6 and 12 months relative to baseline, respectively. Global sleep quality significantly improved over the 12-month intervention (P=.03), but average sleep duration and the prevalence of short sleep duration (<6 h) or poor sleep quality did not change significantly (each P≥.45). Adults with short sleep duration at baseline lost 3.3±0.9% less weight than those with ≥6 h sleep duration (P<.001). Adults with poor sleep quality at baseline lost 1.6±0.8% less weight than those with good sleep quality (P=.04). When considered together, adults with both short sleep duration and poor sleep quality lost at least 5.0% less weight compared with all other sleep duration/quality group combinations (P<.001). Conclusion Our findings highlight the importance of both sleep duration and sleep quality as predictors of behavioral weight loss and suggest that screening for sleep disturbance may be useful to determine who may benefit from additional counseling and resources. Support R01HL077525, K23HL118318