The daily rhythm of sleeping and waking is one of the most fundamental cycles in our lives, and we often equate it with our circadian clock. But the reality is more subtle than that. The circadian clock, as biologists have learned in recent decades, is in fact an incredibly precise molecular machine that exists in nearly every cell in the body. Consisting of a number of proteins that come together and fall apart rhythmically, the clock complex controls the transcription of thousands of genes that affect everything from appetite to cell division. There is a season — or rather, a time of day — for each process that takes its timing from the clock. Sleep just happens to be one such process.