Optimal nap timing and body mass index: beyond duration

Overweight and obesity continue to rise globally, and sleep has emerged as an important behavioral determinant of body weight. While nighttime sleep has been widely studied, the role of daytime napping—especially nap timing—remains unclear. Existing research relies on clock time, which may not reflect individual rhythms. This study investigates whether nap timing referenced to daily events is associated with BMI in two adult populations of habitual nappers (Spain and Brazil). Secondary aims included the association of nap duration and weekend–weekday differences in nap timing and duration with BMI. Habitual nappers (n = 3550) aged 18–65 years were studied. Nap timing was calculated relative to wake time and lunch time. As secondary analyses, nap duration and weekend–weekday differences in nap timing (nap jet lag) and in nap duration were evaluated. Linear regression and restricted cubic spline models were applied. BMI and excess weight prevalence were similar between countries. Compared with Spaniards, Brazilians showed earlier naps (by >2 h), longer weekend naps (by 13 min), and greater nap jet lag (by 23 min) (all p < 0.001). Despite these differences, no significant interactions by country were observed in nap–BMI associations, and data were pooled. Nap timing relative to wake and lunch displayed L-shaped associations with BMI: the highest BMI corresponded to early naps (≈3h after waking and ≈2h before lunch), whereas the lowest BMI (i.e. the inflection points) occurred when naps were 7h18min after wake (β = −0.48 kg/m2 per hour before the inflection point, p = 0.001) and 1h15min after lunch (β = −0.53 kg/m2 per hour before the inflection point, p = 0.031). Shorter and more consistent nap duration across the week was associated with lower BMI (p < 0.05). Findings highlight nap timing as a novel behavioral associated with BMI, warranting further longitudinal research to explore its potential causal role. ClinicalTrials.gov NCT03036592 (ONTIME-MT).