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BACKGROUND: Sleep duration affects metabolic health and appetite regulation, but its role in behavioral weight loss interventions remains unclear as prior studies are limited by small sample sizes, cross-sectional designs, and inconsistent findings.OBJECTIVE: To examine the associations between nighttime sleep duration and weight loss in a behavioral intervention among adults with overweight or obesity in Spain.METHODS: This secondary analysis included adults with overweight or obesity from the ONTIME study, a 25-week behavioral weight loss intervention. Participants self-reported sleep duration at enrollment and were categorized as short (<7 hours), recommended (7-8 hours), or long (>8 hours) sleepers. Outcomes included percent weight loss, clinically meaningful weight loss (≥5%), rate of weight loss (≥0.53kg/week), and attrition. Associations were examined using logistic regression and linear mixed-effects models adjusted for age, sex, baseline weight, intervention duration, and clinic site.RESULTS: Among 3,628 participants (mean age 41.2 ± 14.1 years; 77.6% women), 23.7% reported sleeping >8 hours, 60.9% reported sleeping 7-8 hours, and 15.4% reported sleeping <7 hours per night. Long sleepers had significantly lower average weight loss (7.42%) compared to recommended sleepers (7.90%, P=0.015). Long sleep was associated with 21% lower odds of achieving ≥5% weight loss (OR=0.79, 95% CI: 0.66-0.96), 25% lower odds of rapid weight loss (OR=0.75, 95% CI: 0.63-0.89), and 21% higher odds of attrition (OR=1.21, 95% CI: 1.03-1.43) compared to 7-8h sleepers. No significant associations were observed for short sleepers. Linear mixed-effects models further indicated lower weight loss among long sleepers (β=0.099, SE=0.040, P=0.015), with no significant difference for short sleepers (β=-0.031, SE=0.048, P=0.514).CONCLUSIONS: Habitual long sleep duration was associated with reduced weight loss success and increased risk of attrition in a behavioral weight loss intervention.CLINICAL TRIAL REGISTRY NUMBER AND WEBSITE WHERE IT WAS OBTAINED: ClinicalTrials.gov: NCT02829619 ().