Phase Syndromes, the patient entrains to the 24 hour day, but does so at an intractably abnormal phase. In model organisms, these phenotypes are elicited by mutations affecting circadian period length. A correlation between circadian dysrhythmia and mood abnormalities has been observed in humans for decades. Whether it is the rhythm or the blues, however, that is the primary defect is difficult to disentangle. Work in experimental mammals reveals the basis of this uncertainty.
Stimuli eliciting depression can cause a shift of circadian phase.
Conversely, a circadian clock gene mutation can cause a depression-like syndrome in hamsters. It seems likely, therefore, that mutations in circadian clock genes are one of many causes of affective illness. As a result of the genome sequencing project, human homologs for all known animal circadian clock genes have recently been discovered. Using the latest techniques in the genetics of complex traits, we hope to be able to discern the contribution clock genes make to person to person variation in behavior. The cryb mutation identifies cryptochrome as a circadian photoreceptor in Drosophila.