Recently Domien Beersma, Henk Broer, Kim Gargar, Igor Hoveijn, and I published in Physica D a paper on synchronization and circadian rhythms. The basic idea behind the paper is to study in a simple model how a single pacer cell synchronizes to a periodic signal. This signal includes the effect of the external environment (light-dark cycle) but also the effect of the rest of the pacer cells. It turns out that such system can be described by a family of circle maps. In the paper we discuss the properties of this family (emphasizing resonances and Arnol’d tongues) and their biological significance.