the aim of the present study was to investigate the relationship of sleep quality and time with basal metabolic rate (BMR) and fuel oxidation in basal conditions and during exercise in sedentary middleaged adults. We also studied the mediation role of dietary intake and adherence to the traditional Mediterranean Diet in the relationship between sleep parameters and energy metabolism parameters.A secondary analysis of the FIT-AGEING study was undertaken. 70 middle-aged sedentary adults (40-65 years old) participated in the present study. Sleep quality was assessed using the Pittsburgh Sleep Quality Index (PSQI) and wrist accelerometers (ActiSleep, Actigraph, Pensacola, Florida, USA) for 7 consecutive days. BMR was measured with indirect calorimetry and fuel oxidation was estimated through stoichiometric equations. Maximal fat oxidation was determined by a walking graded exercise test and dietary intake with 24 h recalls. Adherence to the traditional Mediterranean diet was assessed through the PREDIMED questionnaire. PSQI global score (poor sleep quality) was associated with lower basal fat oxidation (BFox), both expressed in g/min and as a percentage of BMR, independently of confounders. We did not find any association between other sleep and energy metabolism parameters. No mediating role of the dietary intake or PREDIMED global score was observed in the association of PSQI and BFox. In conclusion, our study showed that a subjective poor sleep quality was associated with lower BFox, which is not mediated by dietary intake in sedentary adults. Cardiometabolic diseases and obesity are the leading causes of death in developed countries, becoming an epidemic in the last years 1,2. Unhealthy diets represent one of the top risk factors for cardiometabolic diseases and obesity, developing a positive energy balance 3. Simultaneously, a low basal metabolic rate (BMR), an impaired meal-induced thermogenesis and low physical activity levels could result in a reduced total energy expenditure 4. This low total energy expenditure coupled to high energy intake could produce a gradual weight gain and visceral adipose tissue deposition, increasing the risk of cardiometabolic diseases and obesity 5. The ability to oxidize fat as a fuel is considered an important metabolic health parameter 6. An impaired ability to oxidize fat is associated with an increased risk of obesity, type 2 diabetes mellitus, cardiovascular disease, metabolic syndrome, cancer and systemic inflammation 7. Therefore, fat oxidation in basal conditions (BFox) and maximal fat oxidation during exercise (MFO) are considered markers of metabolic health 8-10. Sleep pattern variations, including a decrease in the quality and quantity of sleep, have been shown to be also a risk factor for the development of obesity and cardiometabolic diseases 11. These changes in sleep quality and quantity disrupt the circadian rhythms and may have deleterious consequences on people health 12. Previous studies have provided a causal link between short sleep duration and poor sleep qu...