Tweaking the Human Circadian Clock with Light

The human body contains a clock, located in the brain, that controls the timing of functions such as sleep and body temperature. This clock can be reset by light shining on the eyes so that when days grow longer or shorter, human physiology will adapt appropriately. Now, in a report in the same issue, Campbell and Murphy show that the light signal to reset the clock can be applied by another route, through the skin on the back of the knees. In their commentary, Oren and Terman discuss the ways in which this unexpected mechanism may work and how this finding could affect the way depressed patients are treated with light.