By now you appreciate the powerful effect of light input on circadian rhythms. However, further studies showed that the inner clock can also respond to other kinds of signaling. Some input—notably exercise and melatonin—uses routes other than the eye-to-SCN pathway. Yet other stimuli—notably scheduled food access and methamphetamine—control the timing of inner clocks separate from the SCN.
The food-based clock is a fascinating case, because the signal processing resides in liver tissue. An animal lacking an SCN can still entrain some of its rhythms to a fixed schedule of meals. Considering that there are multiple inner clocks—we’ve described the SCN, as well as the retina and liver—overall rhythmic stability, or strength, requires that all these timing components be synchronized to each other.