Essentials: Timing Light for Better Sleep, Energy & Mood | Dr. Samer Hattar

**Andrew Huberman** (0:00)
Welcome to Huberman Lab Essentials, where we revisit past episodes for the most potent and actionable science-based tools for mental health, physical health, and performance.
I'm Andrew Huberman, and I'm a professor of neurobiology and ophthalmology at Stanford School of Medicine. Today, I have the pleasure of introducing Dr. Samer Hattar as my guest on the Huberman Lab podcast. And now, my conversation with Dr. Samer Hattar. Samer, thanks for sitting down with me.

**Samer Hattar** (0:30)
My pleasure.

**Andrew Huberman** (0:31)
You are best known in scientific circles for your work on how light impacts mood, learning, feeding, hunger, sleep, and these sorts of topics. So, maybe you could just wade us into what the relationship is between light and these things like mood and hunger, et cetera.

**Samer Hattar** (0:49)
Sure. So, I mean, you do appreciate the effect of light for vision. So, when you wake up in a beautiful area, beautiful ocean, light is essential. The sunrise, the sunset. So, that's your conscious perception of light. But light has a completely different aspect that is independent of conscious vision. And that's how it regulates many important functions in your body. I think the best that is well-studied and well-known is your circadian clock. And the word circadian comes from the word circa, which is approximate, and D and E is day. So, it's an approximate day. Why is it an approximate day? Because if I put you or any other human being who have a normal circadian clock in a constant conditions, with no information about feeding time, about sleep time, about what time it is outside, you still have a daily rhythm, but it's not exactly 24 hours. So, it will shift out of the solar day because it's not exactly 24 hours, and hence the name circadian.

**Andrew Huberman** (1:51)
How does that rhythm show up in the tissues of our body?

**Samer Hattar** (1:54)
It shows up at every level that we know we studied. It shows up at the level of the cell, it shows up at the level of the tissue, and it shows up at your behavior. The most obvious for you is your sleep-wake cycle. You sleep and you're awake and sleep at the 24-hour rhythms. The period length of the sleep rhythm on average is 24.2 hours. So you'll be drifting 0.2 hours every day out of the solar day if you don't get the sunlight. So the sunlight adjusts that approximate day to an exact day, so now your behavior is adjusted to the light-dark environment or the solar day. It's part of the brain that is not consciously driven, so you actually do not know when it happens or when it doesn't happen. And that's what we'll get into when I tell you why light affects your mood and why sometimes people don't know how to deal with light to improve their mood, for example.

**Andrew Huberman** (2:46)
What's the relevance? I mean, why should we care about that short difference?

**Samer Hattar** (2:48)
So let's do the math. If you shift out 0.2 hours a day in five days, you're shifting out one hour, so you're literally one hour off in your social behavior in five days. In ten days, you're two hours off. And if you're an organism that is living in the wild, shifting out of the right phase of the cycle, you could either miss food or you could become food. So, it's really essential for survival. I think it's one of the strongest aspects of survival for animals to have the anticipation and the adjustment to the solar cycle.

**Andrew Huberman** (3:20)
What is the machinery that allows that to happen? And how does that machinery work?

**Samer Hattar** (3:27)
Yeah, so we knew that in mammals, including us, we are mammals, humans, that the eyes are required for this function. So if humans are born without eyes or the optic nerves are damaged, humans are not able to adjust to the solar cycle. So we know that the eyes are required. In the human retinas, there are two types of photoreceptors. They are called rods and cones because of their shapes. And these rods and cones simply take the photon energy, which light is made of, and they change it in a way to an electrical signal that allow us to build the image of the environment in our cortices. However, people have found, including me with the work of David Burson and Ignacio Provencio, that there is a subset of ganglion cells. The ganglion cells are the cells that leave the retina, their axon, leave the retina, and project to the brain. So these were supposed to only relay rod and cone information from the light environment to the brain. We found that a small subset of these ganglion cells are themselves photoreceptors that were completely messed in the retina.