Humans organize their lives in an approximately 24-h rhythm, but there are large interindividual differences. Some are morning “Larks” or early risers, mostly active early in the morning, achieving their maximum of mental and physical activity in the morning and becoming tired early in the evening. In contrast, “Owls” usually have difficulties getting out of bed, and need a longer time to have their senses recovered in the morning, but “Owls” are able to work till late evening. Although these differences were often viewed in some kind of dichotomous manner (“Larks” and “Owls”), they also reflect a continuum between both extremes. Chronotype changes during the lifespan: there is a shift toward eveningness during puberty, then back to morningness at the end of adolescence and then people become progressively more morning oriented parallel with an increasing age. Boys and men usually are more evening oriented than girls and women given a similar age range. Chronotype can be measured easily by a variety of questionnaires that have good internal and external validity. Only very few studies have assessed the relationship between chronotype and eating behaviors. Schubert and Randler (2008) found a positive correlation between “Larks” (high CSM scores) and cognitive restraint, and negative relationships between high CSM scores and disinhibition, and between high CSM scores and perceived hunger. By using food logs, Fleig and Randler (2009) revealed that earlier chronotypes (or individuals skewed toward morningness) consumed less fast food, less caffeinated drinks but more dairy products. Concerning eating disorders, bulimia nervosa patients had a sleep onset about 1 h later than healthy individuals. Further, there is evidence for a relationship between eveningness and bulimic behavior in university students. In adolescent girls, correlations between three scales of the Eating Disorder Inventory-2 (EDI-2) and the CSM existed. Individuals scoring higher on eveningness reported higher scores on the drive for thinness, the bulimic, and the body dissatisfaction scale. Also, chronotype was found to be linked with addiction, e.g., with alcohol and cigarette consumption, i.e., evening oriented people smoked more cigarettes and drank more alcohol. Another set of studies showed a significant relationship between sleep duration and obesity, suggesting that short sleepers acquire a higher Body Mass Index. However, it seems that it is the timing of sleeping (or chronotype), rather than the sleep length itself.
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