1. We have previously shown a circadian variation in leucocyte activation and endothelial function which may explain why some inflammatory and vascular diseases show a circadian variation in disease activity/occurrence.

2. We have investigated the circadian variation of two soluble cell adhesion molecules, intercellular adhesion molecule-1 and E-selectin, in 10 healthy volunteers. Soluble intercellular adhesion molecule-1 is released from both activated leucocytes and endothelial cells while soluble E-selectin is released only from activated endothelium.

3. Results show a circadian variation exists for both soluble intercellular adhesion molecule-1 and E-selectin (both P < 0.0001, analysis of variance) with a peak activity at 12:00 h for both measures and a minimum activity at 04:00 h for intercellular adhesion molecule-1 and 00:00 h for E-selectin.

4. These results demonstrate the existence of a diurnal variation in cell adhesion molecules, providing evidence in support of a diurnal pattern in endothelial and leucocyte activation. An alteration in this biological rhythm may help to explain the diurnal variation in disease activity in certain inflammatory and vascular disease states. Furthermore, it stresses the importance of sample time point standardization in clinical studies.

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