Cardiorespiratory Fitness and Circadian Rhythms in Adolescents.

While 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; M age =14.6±2.3 years) completed two study visits on weekdays bracketing an ambulatory assessment during summer vacation. Visit 1 included in-laboratory CRF assessment (peak VO2 ) using a ramp-type progressive cycle ergometry protocol and gas exchange measurement, which 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 dim light melatonin onset (DLMO) phase. All analyses adjusted for sex, pubertal status, and physical activity. Greater peak VO2 was associated with earlier sleep/wake times and circadian phase measures, including acrophase, UP time, DOWN time, LAP time, and chronotype (all p<0.05). Peak VO2 was negatively associated with social jetlag (p=0.02). Additionally, the mixed model analysis revealed a significant interaction effect between peak VO2 and actigraphy-estimated hour-by-hour activity patterns (p<0.001), with the strongest effects observed at around time of waking (6:00-10:00). In healthy adolescents, better CRF was associated with 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.