Wang, R., Zhao, R., Shi, X., & Shi, X. (2003). Circadian influence on change in body fluid during exercise. Medicine and Science in Sports and Exercise, 35(5), Supplement abstract 1376.

The effects of circadian rhythm on body fluid changes in exercise were investigated. Healthy men (N = 16) performed two 1-hr cycling tasks at 5 AM and 5 PM one week apart.

Reductions in plasma volume and body weight were greater in the morning than evening performances. Urine volume and sodium excretion was less in the morning than in the evening.

Implication. Time of day for exercise effects the body fluid response, with dehydration being greater in the morning than in the evening.

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