Complex behaviours are the product of intercellular signalling events, but their intracellular effectors are present in most cell types. The best-studied example of such architecture is the circadian clock, which directs all diurnal behaviour and physiology, and whose central mechanism is present in most body cells. We present below a method to look at its properties via transcriptional reporters virally delivered to primary cells. By studying primary fibroblasts cultivated from skin biopsies in different human subjects, we have been able to analyse the molecular underpinnings of variance in human daily behaviour. Similar methodologies could be applied to other signalling pathways.