Dr. Samer Hattar

We know that they use completely different brain regions.

So the SCN that I told you about earlier, the place where the central pacemaker is, the one that receives direct input from the retina through the IPRGCs to adjust your circadian clock,

is not the area that receives the light input for mood regulation.

It's a completely different brain region.

And what's really amazing, this region also receives direct input from the IPRGCs, but projects to areas in the brain that are known to regulate mood, including the ventral medial prefrontal cortex, which has been studied for many years to be impacted in a human depression.

So just by this amazing serendipity to find that a region that is so deep in the advanced brain, like the prefrontal cortex is your executive brain, one of the most elaborated in humans, to see that they receive input from these ancient photoreceptor was stunning to us.

So that's why we came up with the tripartite model, because as a circadian biologist, I only thought of light through the circadian clock affecting behavior.

As a sleep biologist, they only thought of the homeostatic drive affecting sleep, affecting behavior.

And for people who study light for vision and other things, they thought only of the environmental input.

But now if you put them all together, you get with this tripartite model where it's really mind-boggling and it makes so much sense.

The organism doesn't want to depend on a single component.

But if you could incorporate these three together, you could have a beautiful system that is well-adapted.

So let me tell you the sleep-wake cycle, right?

So we know there is a homeostatic drive to affect sleep.

We've had beautiful talks about that.

Which is basically the longer you're awake, the more you want to be asleep.

So that's your homeostatic drive.

We've talked about the circadian influence of sleep and the fact that light-dark cycle affect the circadian system, which eventually affects sleep.

So these two components are well understood.

Now, the third factor is your direct light or environmental input.