[0:00]Hello, I'm Dr. Andrew Huberman, and I'm a professor of neurobiology and ophthalmology at Stanford School of Medicine. And on this Huberman Lab podcast, I'm going to talk all about light. And specifically, I'm going to talk about light as it relates to your sleep, your wakefulness, your mood, your hormone regulation, and your metabolism. And I'm going to provide you with a tremendous amount of science and science-backed tools. Some of which are completely free and require zero cost, that you can implement immediately in order to optimize each of those things. So by the end of this podcast, you'll have a very clear understanding of how to use light to optimize your sleep, your energy throughout the day, your mood, and your overall physical and mental health. Now, for many of you, when you think about light, you probably just think of it as something that allows you to see. And indeed, it does that. But it also has potent effects on all of your biology, even if you are completely blind, you do not need to be able to consciously perceive light in order for light to have dramatic effects on your physiology. In fact, we know that there are specialized cells in the back of your eye that project to particular so-called non-visual brain areas. And those non-visual brain areas control things like your level of alertness, your sleep/wake cycles, when you get hungry, when you don't get hungry. They control hormone release, and they control your mood. And in fact, many of the mood disorders that currently plague so many millions of people around the world have a strong light component. Things like seasonal affective disorder, for instance, are very well known to be impacted by the amount and intensity and timing of light. And so by the end of this episode, you will learn how to use light from your environment. And believe it or not, the light from your environment, meaning sunlight, moonlight, artificial lights, lights from screens, are absolutely the most powerful lever that you have to adjust your health and well-being. It is more powerful than any drug, it is more powerful than any supplement, it is more powerful than any other behavioral tool, and it is something that is completely readily available to all of us. And again, many of the things that I'm going to describe cost absolutely nothing. So let's talk about how light works to impact your brain and body. The key here is to understand that light is a signal. And it is essentially the biggest signal or zeitgeber, as we call it in science, that sets your so-called circadian clock. You've probably heard of the circadian clock before. It's essentially a set of neurons in your brain that dictate your cycles of sleep and wakefulness. And we often think of those cycles as being set by the circadian clock. And indeed they are. But the truth is the circadian clock itself is set by external cues, and the biggest and most powerful of those is light. So if you're somebody who has trouble sleeping or you wake up in the middle of the night or you just can't seem to get your sleep right, or you're tired during the day and you have energy at night, which is really common nowadays. It's a really good chance that the timing and intensity of light that you're getting at different times of day is simply off and can be adjusted very simply. In order to get your sleep and wakefulness into the proper relationship, which will then in turn impact your metabolism and your hormone systems and your mood. Okay, so let's just jump right into the core mechanisms and then from there we'll derive the various tools. Your eyes are essentially portals by which the outside world, and specifically the light in the outside world, gets access to your brain and sets these so-called clocks. And there are clocks in every cell of your body. But the master clock is located in a structure in your brain called the suprachiasmatic nucleus. You don't need to remember that name, but what's important is that light hits specialized cells in your eye. And this is the key feature that I mentioned earlier, which is that even if you're blind, you still have these cells. These cells are called intrinsically photosensitive retinal ganglion cells. They have a scary name, but essentially what they are are light sensing cells that are distinct from the cells that allow you to see consciously. So they send signals to this master clock in your brain, which then sends signals to every other organ and cell in your body. And it dictates when your cortisol is secreted. Cortisol is that stress hormone that makes you feel alert. It dictates when melatonin is secreted. Melatonin is that hormone that makes you feel sleepy. And it dictates your hunger levels, your levels of growth hormone, etc. So just by understanding that the eyes are sensing light, they're sending those signals to a master clock in your brain, which then dictates all of these other things. You can probably already intuit that if you get light at the wrong times of day, it's going to mess up these systems. And indeed it does. So let's talk about the specific tools that you can start implementing as early as today. And I'm going to start with the most impactful one, and then we're going to layer in additional things. The absolute most important thing that you can do, and if you do nothing else that I suggest on this podcast, please do this, is to view bright light in the morning within 30 to 60 minutes of waking up. Now, it's very important that that light come from sunlight. Okay, so let me explain why. The neurons in your brain that control your sleep and wakefulness and your mood and your hormone cycles and your metabolism are set by the morning light that you perceive. And it is absolutely critical that that light be actual sunlight. So not light through a window, not light through a car windshield, and not artificial light, unless that's your only option, and I'll explain the hierarchy of what to do in that case. But the reason it's so important to get sunlight in your eyes early in the day is that it sets a timer for the release of melatonin later that night.