Research Section for Behavioral Regulation and Health, Department of Psychiatry, Perelman School of Medicine at the University of Pennsylvania, 1016 Blockley Hall, 423 Guardian Drive, 19104, Philadelphia, USA. astahn@pennmedicine.upenn.edu. Charité - Universitätsmedizin Berlin, a corporate member of Freie Universität Berlin, Humboldt-Universität zu Berlin, and Berlin Institute of Health, Institute of Physiology, Berlin, 10117, Germany. astahn@pennmedicine.upenn.edu. Charité - Universitätsmedizin Berlin, a corporate member of Freie Universität Berlin, Humboldt-Universität zu Berlin, and Berlin Institute of Health, Institute of Physiology, Berlin, 10117, Germany. Department of Biomedical Sciences for Health, Università degli Studi di Milano, via G. Colombo 71, 20133, Milan, Italy. Department of Sport Science, University of Stellenbosch, Private Bag X1, Matieland, 7602, South Africa.