Cardiovascular reactivity to stressors: effect of time of day?

Reduced cardiovagal baroreflex sensitivity and a peak in the incidence of cardiovascular events in the hours immediately after waking from nocturnal sleep suggest that cardiovascular control is impaired in the morning compared with other times of day. Previous research indicates that diurnal variation exists in acute blood pressure (BP) responses to exercise. However, the effect of time of day on activities such as cognitive tasks and "passive coping" physical tasks has yet to be established. Therefore, the primary aim of this study was to explore cardiovascular responses to physical and mental stressors at two times of day that have previously been associated with differing levels of cardiovascular control. In addition, the effect of the chronotype was examined to identify possible interactions between morningness/eveningness, time of day and responses to stressors. Fourteen healthy, young subjects completed a morning (08:30 h) and an afternoon (13:30 h) trial on separate days. Subjects performed a mental arithmetic task and a cold pressor test while beat-to-beat measurements of BP and heart rate were recorded continuously. Reactivity was determined using mean change scores in systolic BP, diastolic BP, mean arterial pressure, heart rate and rate-pressure product (RPP) from a period of rest recorded immediately prior to the task. There was no significant difference in cardiovascular reactivity between the morning and afternoon (p > 0.05). The time course of the responses and subsequent recovery were also consistent between the two times of day (p > 0.05). There was a significant interaction between time of day and chronotype, although this was apparent only for heart rate and RPP reactivity (p < 0.05); subjects tending towards "morningness" exhibited greater heart rate and RPP reactivity in the afternoon, and subjects tending towards "eveningness" exhibited greater heart rate and RPP reactivity in the morning. No interactions were observed between time of day and chronotype for BP reactivity (p > 0.05). Despite effects of time of day on heart rate that are dependent upon chronotype, this study suggests that BP control during mental and passive physical stress is not altered between the morning and afternoon.