Your genome is the same right now as it was yesterday, last week, last year, or the day you were born. But your microbiomes—the combined genes of all the trillions of microbes that share your body—have shifted since the sun came up this morning. And they will change again before the next sunrise.

These cycles are a lot like our own body clocks, or circadian rhythms. Over a 24 hour period, the levels of many molecules in our body rise and fall in predictable fashion. These rhythms affect everything from our body temperature to our brain activity to how well we respond to medicine. But these clocks tick by themselves. You can reset them by exposing yourself to light at