Morning increase in whole blood viscosity: a consequence of a homeostatic nocturnal haemodynamic pattern

In a series of studies, we have shown that in non‐human primates there is a consistent overnight fall in cardiac output and central venous pressure, and a rise in total peripheral resistance. This haemodynamic pattern is associated with a higher haematocrit level in the morning suggesting that these changes in the circulation are homeostatic adjustments to a nighttime fall in plasma volume. The present study was designed to test the hypothesis that in the morning whole blood viscosity also is higher. Whole blood viscosity was measured at shear rates of 450, 225, 90, 45, and 22.5 s‐^l in each of six monkeys, on four occasions, at 2‐week intervals, at 17.00 and 09.00 h the next morning. The average haematocrit was 4.2% higher in the morning than in the previous evening (P < 0.01). Viscosity decreased monotonically at progressively higher shear rates but was always significantly higher in the morning than in the evening (P < 0.01 at all shear rates). When viscosity was adjusted by covarying for haematocrit level, the morning/evening differences became non‐significant. However, the morning/evening differences in linear trend of shear stress as a function of shear rate persisted. These findings add further support to our hypothesis that the nocturnal haemodynamic pattern in non‐human primates is related to a reduction in plasma volume, and they also suggest that the morning rise in haematocrit is a major contributing factor to the elevated viscosity.