Author(s): BRADSHAW, L., HUTCHINS, K.M., DAVIS, M., MACPHERSON, S., DIBNER, C., GONZALEZ, J.T., HAWLEY, J., JOHNSTON, J., KOUMANOV, F., MAHER, J., PERKIN, O.J., STOCKS, B.N., THOMPSON, D., TSINTZAS, K., VAN LOON, L.J.C., WALHIN, J.P., WILLIAMS, S., ZIERATH, J.R., BETTS, J.A. , Institution: UNIVERSITY OF BATH, Country: UNITED KINGDOM, Abstract-ID: 2331

INTRODUCTION:
Circadian rhythms align many physiological processes and can be influenced by environmental and behavioural cues such as light-dark, feeding-fasting or activity-rest (1). Blood cell counts exhibit a daily rhythm, with different cell types having peaks and troughs at different times of day (2). An acute bout of exercise causes an immune response with a transient rise in total white blood cells (WBC), neutrophils, lymphocytes and monocytes, with higher intensity exercise causing a greater increase in cell counts (2). However, it is currently unknown whether the timing of an acute bout of exercise (morning versus evening) influences the initial immune cell response to the exercise or the overall diurnal rhythm in blood cell counts following exercise.
METHODS:
In this interim analysis, twelve participants (7 males and 5 females) completed one of three conditions in a parallel design with hourly blood samples after 7 days of light and sleep standardisation. In the CONTROL condition participants rested for 24 hours starting at 0800 h. In the EARLY and LATE conditions participants performed 1 hour of moderate intensity cycling (50 % delta of lactate threshold 1 and 2) starting at 0800 h in EARLY and 2000 h in LATE followed by 23 hours of rest. Whole blood samples were analysed for concentrations of haemoglobin and counts of platelets, WBC, neutrophils, lymphocytes, monocytes, eosinophils and basophils. Differences between conditions were analysed by two-way ANOVA.
RESULTS:
There was a time by group interaction of WBC (p = 0.05) and lymphocyte counts (p = 0.01). with significant effects of time (WBC p = 0.03, lymphocyte p = 0.001). There was also a significant effect of time of neutrophil count (p = 0.02). The group by time effects seem to be driven by an increase in counts at the completion of exercise but there were no group effects suggesting there is no difference in WBC, lymphocyte or neutrophil counts over the 23 hours following exercise. There were no significant differences between groups in concentrations of haemoglobin or counts of platelets, monocytes, eosinophils or basophils (time x conditions interactions p > 0.05).
CONCLUSION:
An acute bout of moderate intensity cycling increased counts of WBC which appears to be due to higher counts of lymphocytes irrespective of morning or evening exercise. The time of day of an acute bout of moderate intensity cycling does not alter the acute immune cell response or cell counts over the following 23 hours.

References
1. Betts JA, Bowden Davies KA, Smith HA, Hawley JA. Physiological rhythms and metabolic regulation: Shining light on skeletal muscle. Exp Physiol 110: 976–983, 2025. doi: 10.1113/EP091890.
2. Feriel J, Tchipeva D, Depasse F. Effects of circadian variation, lifestyle and environment on hematological parameters: A narrative review. Int J Lab Hematol 43: 917–926, 2021. doi: 10.1111/ijlh.13590.