Your body’s stress hormone, cortisol, goes up and down based on the time of day and whether you’re in light or dark, not just because you’re stressed.

Even though the mice were exposed to the same light schedule, eating too much fatty food messed up their body’s daily cortisol rhythm, showing that what you eat can interfere with your body’s internal clock.

Even when scientists messed up the cows' light schedule, their cortisol levels still rose and fell at the same time each day—showing that light, not just food or stress, is the main clock that controls cortisol.