 Now why are circadian rhythms so crucial? Well, think of these deaf, dumb and blind movers trying to move a sofa. They have no ears and mouths so they can't talk to each other, but they need to lift at the same time, they need to walk in the same direction, or else the sofa is going to fall down. So they need to coordinate their activities, but if they can't communicate, how do they do it? And our cells are in that situation. Our cells need to cooperate with all the other cells. They need to achieve biological things that they can't do alone. They need to do it with a cluster of other cells, like an organ. Everybody needs to be working together, but they need to all do it at the same time. They need to all be running the same program. And how do they achieve that? They achieve it through clocks. And they all have to have their clocks synchronized. If they don't have their clocks synchronized, then they won't do it. So all of our cells, they've got a biological program. At this time of day, I'm going to do this. And if all of our cells have the same time, then they'll be able to interact with each other successfully, and all of our bodily functions will be executed well. How do we give time to those clocks? How do we get all the clocks ticking and all of them synchronized? Well, there are five main ways. One of them is light exposure. One is exposure to ambient temperature. One is through social interactions. One through exercise. One through meal timing. So let me talk a little bit more about each of those. How does light give us our timing? Well, it turns out when light, mostly blue light, but also ultraviolet light and a little bit of green, when it strikes our retina, there are some special receptors that send signals to a part of the brain called the suprachiasmatic nucleus, which is where the so-called central clock is located. So it's the single most important clock in the body. And that portion of the brain sends signals out to the rest of the body. This is what I think the time is. So that's a very important clock. And what drives it? Blue light is really crucial. And everything from about here on, yellow up, can affect it somewhat, but basically red, orange, and yellow with no green has no effect on the central clock. So what does that mean? In the nighttime, you can use red, orange, yellow light with no problem. It won't have any effect on your circadian rhythms. It won't tell your brain that it's daytime. But in the daytime, you really need all this blue light. So whenever you're exposed to blue light, it's telling your body it's daytime. So by day, you need to get sunlight. And when you're indoors during the day, you need to expose yourself to bright, full-spectrum lighting. Now what kind of lighting do people actually buy? If you look at these lights, you'll see they have a yellowish tinge to them. And that's the worst possible kind of lighting to buy. Don't buy anything that says cool white bulb. That's uncool. What you want is something with a color temperature of at least 5,500 Kelvin. And it should look a little harsh. It should look very bright, very blue. Think of the halogen headlights. And what should things look like at night? They should look like a campfire. Color temperature more like 600 Kelvin. And it's very red, very yellow, and very little blue. So these yellow lights, they have too much blue, they'll disrupt your night, but they don't have enough blue to untrain your day. Now it turns out, since Thomas Edison invented the light bulb, he's a great man in many ways, but he really damaged our health with that one. So we now have these light bulbs all around us at night. And it turns out that having all that light at night is probably disrupting our health. And so, for instance, light exposure at night is closely tied to obesity rates. So in this study, they split light exposure up into three groups. A low light group, middle light third, and a high light third. And the darker the light. Basically, the people with middle amounts of light exposure at night had 10% less obesity than people with highest light and people with the lowest light exposure had 20% less obesity. And we know that circadian rhythm disruption causes obesity in animals. So this is very consistent with what we know. Alright, temperature, really important. We want to be exposed to warmer temperatures in the day, cooler temperatures at night. Now, in our natural environment, outdoors, that happens automatically. There's no place in the world you can go where it's hotter at night than in the day. But now that we have artificial control over environments, what do we do? Especially if you're in the South, you put air conditioning on all day. So it's cool. And you go through offices, the mall, everything's very cold. And then you get home and you want to save money on energy and so you turn the air conditioning off and now it gets warm. So you've reversed the day-night cycle. Food, extremely important, circadian rhythm and trainer. It's very important to do intermittent fasting, have an extended overnight fast. You should never eat any calories at night and you should have at least a 12-hour night. So once it's nighttime for you, don't eat any more calories and you should actually extend the fast longer than 12 hours. So I generally do about 16. And I think that's about optimal.