Your brain, your heart -- in fact, every cell in your body -- has its own clock telling you when to be alert and when to pack it in. You probably know that jet lag and daylight savings time affect that clock, but did you know that the food you eat (and when you eat it) as well as your activity level can also wreak havoc on it? Emily Manoogian, PhD, chronobiologist and clinical researcher at the Salk Institute for Biological Studies in San Diego, explains how shift work, long flights, eating at the wrong times, and even staying up too late on weekends all affect health, mood, and emotional regulation.

Plus - what happens when animals are thrown off their rhythm?