Objective The aim of this study was to determine the association between siestas/no siestas and obesity, considering siesta duration (long: >30 minutes, short: ≤30 minutes), and test whether siesta traits and/or lifestyle factors mediate the association of siestas with obesity and metabolic syndrome (MetS). Methods This was a cross‐sectional study of 3275 adults from a Mediterranean population (the Obesity, Nutrigenetics, TIming, and MEditerranean [ONTIME] study) who had the opportunity of taking siestas because it is culturally embedded. Results Thirty‐five percent of participants usually took siestas (16% long siestas). Compared with the no‐siesta group, long siestas were associated with higher values of BMI, waist circumference, fasting glucose, systolic blood pressure, and diastolic blood pressure, as well as with a higher prevalence of MetS (41%; p = 0.015). In contrast, the probability of having elevated SBP was lower in the short‐siesta group (21%; p = 0.044) than in the no‐siesta group. Smoking a higher number of cigarettes per day mediated the association of long siestas with higher BMI (by 12%, percentage of association mediated by smoking; p < 0.05). Similarly, delays in nighttime sleep and eating schedules and higher energy intake at lunch (the meal preceding siestas) mediated the association between higher BMI and long siestas by 8%, 4%, and 5% (all p < 0.05). Napping in bed (vs. sofa/armchair) showed a trend to mediate the association between long siestas and higher SBP (by 6%; p = 0.055). Conclusions Siesta duration is relevant in obesity/MetS. Timing of nighttime sleep and eating, energy intake at lunch, cigarette smoking, and siesta location mediated this association.