ArticleSleep2026
Nap Timing Matters: Later Siesta Associates with Increased Muscle Mass and Decreased Fat Mass in Spain and the US.
Article in Sleep, 2026. The graph could read no effect estimate from its abstract, so it casts no vote on the map. It is linked to 2 registered trials, which are not on this map. Not yet cited in PubMed.
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The abstract states no effect estimate the extractor could read, or names no intervention and outcome on the map, so this paper lights no cell and moves no belief. It is still indexed, cited and linked below.
The trial behind it
Trials whose registry record cites this paper, or whose number appears in the abstract. A trial that started after this paper was published is citing it as background, not reporting it.
Meal Timing, Genetics and Weight Loss in a Mediterranean Population
Shift Work, Heredity, Insulin, and Food Timing (SHIFT) Study
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Authors and funding
6 authors.
Funding
Abstract
STUDY
objectivesPrior 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.
methodsActigraphy-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.
resultsIn 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).
conclusionsLater 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.
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Registered trials
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