Although the issue is still being explored, circadian clocks may have arisen independently four times during evolution, highlighting how important this timekeeping mechanism is for the survival of organisms. The first circadian gene identified in mice, Clock , was discovered by making mutations in mice and looking for animals with altered rhythms. Mice also have their own version of Drosophila 's timeless gene, as well as two Cry genes. A new circadian gene, first identified in the tau hamster mutant, was recently cloned in mice and it codes for the enzyme casein kinase 1 epsilon. The overall structure and function of the human clock is likely to be very similar to the structure and function of the mouse clock.
Circadian rhythms are very significant in human health, most obviously as they affect sleep.
There are genetic disorders in humans that apparently disrupt circadian rhythms and may represent mutations in genes related to those identified in mice and fruit flies.