 Now all six of these things share a common feature. They entrain circadian rhythms. Circadian means on a 24-hour cycle like the sun, the day-night cycle. And all of these things are meant to be rhythmic parts of our lives. So what happens? Our body has lots of clocks. Every cell has a circadian clock, its own internal clock. We have a trillion cells in our body. All of those trillion clocks need to be synchronized in order for us to have good health. So we have to make sure each of those cellular clocks is working. It's not like the grandfather clock that stopped ticking. And then we have to make sure they're all keeping the same time. It's like in the spy movies. All the spies have to get together, synchronize their watches. If they're going to work together successfully, if our cells are going to work together successfully, they have to synchronize all of their clocks. And if you achieve that, then you'll gain at least six years of life expectancy. So that's a really big gain. What happens if you fail to do it? Well, we have all these different ways of doing it. And it turns out just screwing up any one of them is pretty terrible. So if you look at people who get sleep loss, either because they do night shift work or they have sleep apnea, which obstructs their breathing and wakes them in the middle of the night, here's the list of diseases they get and they lose about six years off their lifespan. But a totally different way of entraining circadian rhythms is exercise. That's something we need to do in the daytime, and it establishes good circadian rhythms. And what about people who never exercise? What happens to them? Well, they lose six years off their lifespan and they get exactly the same list of diseases as the people who do night shift work or have sleep apnea. Why is that? These things are feeding into the same biological pathways. And that's where they deliver their benefits.