Although cardiorespiratory fitness (CRF), an important marker of youth health, is associated with earlier sleep/wake schedule, its relationship with circadian rhythms is unclear. This study examined the associations between CRF and rhythm variables in adolescents. Eighteen healthy adolescents (10 females and 8 males; Mage = 14.6 ± 2.3 yr) completed two study visits on weekdays bracketing an ambulatory assessment during summer vacation. Visit 1 included in-laboratory CRF assessment (peak V̇o2) using a ramp-type progressive cycle ergometry protocol and gas exchange measurement, which was followed by 7-14 days of actigraphy to assess sleep/wake patterns and 24-h activity rhythms. During Visit 2, chronotype, social jetlag (i.e., the difference in midsleep time between weekdays and weekends), and phase preference were assessed using a questionnaire, and hourly saliva samples were collected to determine the dim light melatonin onset (DLMO) phase. All analyses were adjusted for sex, pubertal status, and physical activity. Greater peak V̇o2 was associated with earlier sleep/wake times and circadian phase measures, including acrophase, UP time, DOWN time, last activity peak (LAP) time, and chronotype (all P < 0.05). Peak V̇o2 was negatively associated with social jetlag (P = 0.02). In addition, the mixed-model analysis revealed a significant interaction effect between peak V̇o2 and actigraphy-estimated hour-by-hour activity patterns (P < 0.001), with the strongest effects observed at around the time of waking (0600-1000). In healthy adolescents, better CRF was associated with an earlier circadian phase and increased activity levels notably during the morning. Future studies are needed to investigate the longitudinal effects of the interactions between CRF and advanced rhythms on health outcomes.NEW & NOTEWORTHY In healthy adolescents, better cardiorespiratory fitness, as assessed by the gold standard measure [laboratory-based assessment of peak oxygen consumption (V̇o2)], was associated with earlier circadian timing of sleep/wake patterns, rest-activity rhythms and chronotype, and less social jetlag. These findings highlight the close interrelationships between fitness and rhythms and raise the possibility that maintaining higher cardiorespiratory fitness levels alongside earlier sleep/wake schedule and activity rhythms may be important behavioral intervention targets to promote health in adolescents.
Keywords: adolescence; aerobic fitness; circadian activity rhythm; circadian rhythms; sleep.