Circadian variation in human peripheral blood flow levels and exercise responses.

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Abstract
The effect of time of day upon forearm arterial blood flow, both at rest and immediately following a sustained submaximal contraction of the forearm muscles, was studied in eight male subjects living a normal routine. Data on blood flow, heart rate, and oral temperature were collected at 0900, 1130, 1500, and 2230 hr. from subjects resting 30 min. prior to each experiment. Circadian patterns in both resting forearm arterial flow and maximum postexercise flow were obtained, the rhythms generally peaking in the late afternoon as did heart rate and body temperature. Circadian variation magnitudes were approximately 45% of their means for both resting and postexercise arterial flows. Shifting resting levels of arterial flow with leg negative pressure caused a decrease in all flow levels but did not alter the circadian pattern. The magnitude of these circadian variations appears sufficient to indicate a cyclic pattern in peripheral resistance in addition to known arterial pressure rhythms.