Train hard, sleep well? Perceived training load, sleep quantity and sleep stage distribution in elite level athletes - Knufinke, M.; Nieuwenhuys, A.; Geurts, S. A. E.; Møst, E. I. S.; Maase, K.; Moen, M. H.; Coenen, A. M. L.; Kompier, M. A. J. (Journal of Science and Medicine in Sport, 2018)