The circadian system entrains behavioral and physiological rhythms to environmental
cycles, and modern lifestyles disrupt this entrainment. We investigated a timed exercise
intervention to phase shift the internal circadian rhythm. In 52 young, sedentary
adults, dim light melatonin onset (DLMO) was measured before and after 5 days of morning
(10 hours after DLMO; n = 26) or evening (20 hours after DLMO; n = 26) exercise.
Phase shifts were calculated as the difference in DLMO before and after exercise.
Morning exercise induced phase advance shifts (0.62 ± 0.18 hours) that were significantly
greater than phase shifts from evening exercise (–0.02 ± 0.18 hours; P = 0.01).
Chronotype also influenced the effect of timed exercise. For later chronotypes, both
morning and evening exercise induced phase advances (0.54 ± 0.29 hours and 0.46 ±0.25
hours, respectively). In contrast, earlier chronotypes had phase advances from morning
exercise (0.49 ± 0.25 hours) but had phase delays from evening exercise (–0.41 ± 0.29
hours). Late chronotypes — those who experience the most severe circadian misalignment
— may benefit from phase advances induced by exercise in the morning or evening, but
evening exercise may exacerbate circadian misalignment in early chronotypes. Thus,
personalized exercise timing prescription, based on chronotype, could alleviate circadian
misalignment in young adults. Trial registration can be found at www.clinicaltrials.gov
(NCT04097886). Funding was supplied by NIH grants UL1TR001998 and TL1TR001997, the
Barnstable Brown Diabetes and Obesity Center, the Pediatric Exercise Physiology Laboratory
Endowment, the Arvle and Ellen Turner Thacker Research Fund, and the University of
Kentucky. Exercise, when performed in the morning, phase shifts the internal circadian
rhythm and has the potential to alleviate circadian misalignment.