The influence of exercise timing on cardiometabolic, biochemical and physiological outcomes: a chronotype-informed approach

Abstract

Exercise is recognised as a potent strategy against non-communicable disease prevention and treatment. Indeed, abundant literature exists concerning exercise intensity, duration, and type; however, less is known regarding the impact time of day has on physiological, clinical, and performance responses to exercise. Considering the growing research interest and knowledge in circadian rhythms, the area of exercise timing requires further attention to elucidate whether enhanced exercise responses can be stimulated at different times of day. Undoubtedly, this could benefit exercise scientists, medical practitioners, and sports coaches alike for the respective application these findings could have towards patients and athletes. Therefore, the main aim of this thesis is to examine the impact exercise timing has on markers of physiological and clinical interest.

A meta-review was conducted initially to assess whether the time of day influences physiological responses (V̇O2max/peak, vascular glucose, and blood pressure) following exercise. Single exercise bouts (acute), and training programmes (chronic) performed in the morning, and in the afternoon/evening were included. There were a total of 625 participants included in this meta-review and findings showed that afternoon/evening exercise resulted in significantly greater reductions than morning exercise for several blood pressure markers (acute and chronic) and also for blood glucose (chronic – specifically in participants with Type 2 Diabetes Mellitus (T2DM) and/or overweight and obesity). Consequently, this meta-review identified research gaps (e.g., a lack of grouping and analysis by chronotype, whereby chronotype refers to an individual’s time-of-day preference) that paved the way for experimental studies in this thesis.

In study 1, nine aerobically untrained males (27.6 ± 5.7 years; 176.3 ± 5.6 cm; 90.6 ± 15.6 kg) underwent a randomised crossover trial. The study consisted of participants performing a single V̇O2peak test on the cycle ergometer in both the morning and afternoon (separated by a one-month washout period). The test itself was used as a measure of aerobic performance, and further measures were taken immediately post-exercise. These included blood pressure, blood glucose, blood lipids, tumour necrosis factor-a (TNF-α), lipid hydroperoxides (LOOH), and superoxide dismutase (SOD). The findings showed no statistically significant interaction effects (p>0.05). In all, this study suggests that exercise timing has no influence on acute exercise responses.

A second study was conducted using the same participants as Study 1, whereby after the first V̇O2peak test in both exercise timing groups, the participants proceeded to undergo a two-week training programme. This consisted of high-intensity interval training (HIIT) (four sessions at ~90% V̇O2peak) and one final V̇O2peak test. Training programmes were separated by a two-week washout period. The same measurements as Study 1 were also used in addition to sleep using the Pittsburgh Sleep Quality Index (PSQI). Exercise per se resulted in significant reductions in systolic and mean arterial blood pressure, and total cholesterol (p<0.05, respectively). Interestingly, chronotype-specific statistical changes were seen in fat %, muscle %, and V̇O2peak. These measures underwent greater beneficial changes when exercise timing aligned with participant chronotype (assessed using the Morning-Eveningness Questionnaire (MEQ)).

The final study involved assessing the influence of time-of-day exercise (a.m. vs. p.m.) in 24 inactive and untrained adults (20 males / 4 females) (29.8 ± 10.6 years old; 176.1 ± 7.4 cm; 80.8 ± 12.4 kg). Each group consisted of 12 participants and within this six were morning and six were intermediate chronotypes. A six-week walking intervention was used (150 minutes of walking per-week at 65% maximum heart rate (HRmax)) to investigate the same markers as Study 2, in addition to physical tests (six-minute walk test, handgrip strength test, and sit-to-stand test). Similar to Study 2, significantly beneficial improvements were observed from baseline to post-intervention testing for systolic blood pressure, as well as fat %, muscle %, and the six-minute walk test (p<0.05, respectively). In this study, low-density lipoprotein cholesterol (LDL-C) endured significantly greater reductions following morning exercise (p<0.05). Furthermore, LDL-C responded favourably when exercise timing was aligned with chronotype, whereas, mean arterial blood pressure responded better when misaligned (p<0.05).

Date of Award	Oct 2025
Original language	English
Sponsors	Department of Education
Supervisor	Gerard McMahon (Supervisor), Conor McClean (Supervisor), Jason Wilson (Supervisor) & Gareth Davison (Supervisor)