It is well known that clock genes such as Per1 and Per2 operate molecular clock works of circadian system in mammals. Clock gene expression is observed not only in the suprachiasmatic nucleus (SCN), a main oscillator, but several brain nuclei such as hippocampus, amygdala and cerebral cortex. Thus, clock genes that appeared in the brain must possess some role in brain function, for example clock system may play a critical role in clock function such as clock and time perception.
There are two kinds of circadian clock; SCN-dependent and SCN-independent circadian rhythms. The former rhythm is reset by environmental light-dark signal, and the latter is reset by daily feeding with fixed time during daytime. Under restricted feeding conditions, the peak of circadian rhythm of Per1 and Per2 genes in the cerebral cortex and liver was advanced for 4-6hrs. This result strongly suggests that cerebral circadian rhythm of clock gene expression is sensitive to time cue induced by feeding schedule rather than light-dark schedule. Internal clock system governed by SCN-dependent or SCN-independent circadian rhythm may be used for reference of clock perception, time perception or time estimation.