134 In many cases it can be cured simply by the administration of strong artificial light during early morning hours.135, 136 The successful treatment of SAD with light suggests that this mood disorder is caused by an impairment of circadian clock synchronization, either because of insufficient luminosity or deregulated melatonin secretion during wintertime.134 In addition to the serious physical and psychic illnesses mentioned above, there are more innocuous manifestations Inhibitors,research,lifescience,medical of aberrant circadian clock functions. Human subjects have individual preferences for their activity phase and, accordingly, can be classified into

“chronotypes.”137 Due to socioeconomic constraints many chronotypes can only adopt their favorite lifestyle during weekends and vacations.138 “Morning larks” choose to get Inhibitors,research,lifescience,medical up early in the morning and go to bed relatively early at night, while “night owls” selleck inhibitor prefer to stay in bed longer and to remain active during a good part of the night. The most extreme forms of these behaviors are known as advanced sleep phase syndrome (ASPS) and delayed sleep phase syndrome (DSPS), respectively.139 In one form of familial advanced sleep phase syndrome (FASPS) a mutation in Inhibitors,research,lifescience,medical the hPER2 gene was identified as the culprit.140 The mutant hPER2 protein carries a glycine residue instead of a serine

residue at position 662. This mutation prevents a phosphorylation, normally occurring on S662, which triggers further phosphorylation by casein kinases Inhibitors,research,lifescience,medical 1S/ε (CK18 and CK1ε) at nearby serine residues C-terminal to S662.

In the absence of these phosphorylations, mPER2 accumulates to lower than normal levels, resulting in a shortening of the period length and, as a consequence, in a daily phase advance. These molecular events could be successfully reproduced in transgenic mice141 and cultured fibroblasts141 expressing transgenes specifying S662G mutant proteins. The successful dissection Inhibitors,research,lifescience,medical of molecular mechanisms responsible for FASPS in animal and even below cellular model systems exemplifies the power of reductionist approaches in tackling seemingly complex behavioral traits. Conclusions Although the first circadian clock was discovered almost 280 years ago, the mechanisms involved in biological timekeeping remained a mystery for the following two and a half centuries. Owing to the development of powerful genetic, genomic, and molecular tools during the past few decades, clock genes were able to be identified, isolated, and studied in several model systems. These technical advances converted circadian rhythm research from a purely phenomenological to a molecular and mechanistic discipline. In one organism, cyanobacteria, a temperature-compensated clock ticking for over a week could be reconstituted with purified recombinant proteins in the test tube.