When it’s dark, and we start to fall asleep, most of us think we’re tired because our bodies need rest. Yet circadian rhythms affect our bodies not just on a global scale, but at the level of individual organs, and even genes. Now, scientists at the Salk Institute have determined the specific genetic switches that sync liver activity to the circadian cycle. Their finding gives further insight into the mechanisms behind health-threatening conditions such as high blood sugar and high cholesterol.