Elite athletes travelling to international destinations >5 time zone differences from their home country have a 2-3-fold increased risk of illness - Schwellnus, M. P.; Derman, W. E.; Jordaan, E.; Page, T.; Lambert, M. I.; Readhead, C.; Roberts, C.; Kohler, R.; Collins, R.; Kara, S.; Morris, M. I.; Strauss, O.; et al. (British Journal of Sports Medicine, 2012)