For a year, representatives of the research group of the Department of Human Nutrition at the Department of Biochemistry and Biotechnology of the Rovira and Virgili University (Spain) tracked changes in weight and distribution of adipose tissue in 1,986 people. These were all suffering from overweight, obesity and metabolic disorders. Throughout the year, all participants of the study worked under an intensive weight loss program based on the Mediterranean diet, increased physical activity and behavioural therapy. During the observations, the researchers noticed that participants who slept less than six hours a night and whose sleep patterns were generally unstable (different number of hours of sleep) not only lost weight more slowly, compared to others, but also lagged in reducing their waist sizes.