Influence of Time of Day on Skin Microvascular Response to Post-Occlusive Reactive Hyperemia

Study aim: Circadian rhythms influence numerous physiological processes, including vascular regulation and skin microcirculation. However, evidence regarding diurnal variability in post-occlusive reactive hyperemia (PORH) parameters remains limited. The aim of this study was to investigate the effect of time of day on skin microcirculatory responses assessed using laser Doppler flowmetry during a PORH test in healthy adults. Material and methods: Thirty healthy volunteers (15 women and 15 men) participated in the study. Skin microcirculation was assessed twice on the same day using laser Doppler flowmetry: in the morning (08:00-10:00) and in the evening (18:00-20:00). The analyzed parameters included resting flow (RF), biological zero (BZ), peak perfusion (PP), time to peak (TTP), and recovery time (TTR). A standardized occlusion protocol (100% arterial occlusion pressure for 3 min) was applied, followed by 4 min of post-occlusive monitoring. Data distribution was assessed using the Shapiro-Wilk test. Due to non-normal distribution, paired comparisons were performed using the Wilcoxon signed-rank test. Statistical significance was set at p < 0.05. Results: Resting flow was significantly higher in the evening compared with the morning (p = 0.038), indicating moderate diurnal variability in basal skin blood flow. No significant differences were observed for BZ, PP, TTP, or TTR (p > 0.05). Conclusions: Time of day influences resting skin blood flow but does not affect PORH parameters in healthy adults. These findings suggest that basal microvascular tone shows diurnal variability, whereas the reactive microcirculatory response remains stable. Therefore, measurement timing should be considered when interpreting resting microcirculatory assessments.