Figure 4.

Hierarchical organization of circadian rhythms. Environmental time cues calibrate the central circadian pacemaker, which organizes regulatory structures like the hypothalamus and pineal gland, ultimately synchronizing the expression of clock genes throughout the body. At the cellular level, circadian rhythms are coordinated by the network of core clock proteins (see Figure 2). Without signals from the central pacemaker, organs and systems can uncouple into free-running rhythms. SCN = suprachiasmatic nuclei.