Nap Timing Matters: Later Siesta Associates with Increased Muscle Mass and Decreased Fat Mass in Spain and the US

Study objectives Prior studies on siesta and obesity are inconsistent, and the role of siesta timing remains unclear. We examined cross-sectional associations between objectively estimated siesta characteristics and body composition in two populations across two continents. Methods Actigraphy-derived siesta timing (clock time and relative to wake/bedtime), duration, and frequency from 90 adults in the Spanish ONTIME Study (NCT02829619) and 69 adults in the US SHIFT Study (NCT02997319) were analyzed. Outcomes included BMI, waist circumference, body fat percentage, and muscle mass percentage. Associations were assessed using adjusted linear and logistic models. Results In the ONTIME Study (52±10 years, 91% women), each 1-hour later siesta start time was associated with lower BMI (β(SE)=−0.044(0.012) log(kg/m2), p=0.0003), body fat (β(SE)=−0.031(0.011)log(%), p=0.014), waist circumference (β(SE)=−0.026(0.012)log(cm), p=0.009), and higher muscle mass in women (β(SE)=0.019(0.006)log(%), p=0.005). Participants with late siestas (16:12–17:42 h) had ~3.94 kg/m2 lower BMI vs. early siestas (ANCOVA P=0.017). Longer siesta duration was associated with 3.98 higher odds of overweight/obesity (95% CI:1.12–14.18, p=0.033), while higher siesta frequency (days/week) trended toward lower body fat and higher muscle mass (p<0.1). Analyses in the independent SHIFT Study (31±9 years, 62.5% women) were directionally consistent but not statistically significant. Fixed-effect meta-analysis combining both cohorts (n = 159) identified significant associations between later siesta start time and lower BMI, lower body fat percentage, lower waist circumference and higher muscle mass percentage (all p<0.05). Conclusions Later siesta timing is associated with lower obesity traits and higher muscle mass. Objective multidimensional assessment of siesta may help identify behavioral patterns associated with body composition and inform future intervention studies on weight management.