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

In this Huberman Lab Essentials conversation, Andrew Huberman sits down with Samer Hattar, a chronobiologist at the National Institutes of Mental Health whose research on light and mood frames the entire discussion. Hattar's starting point is a distinction most people never consciously notice: light does two different jobs in the body. One is vision, the conscious perception of sunrises and sunsets. The other is a completely non-visual function, the regulation of the body's internal timing systems, which, as Hattar puts it, is "a completely different aspect that is independent of conscious vision" [s00004].

That second job is carried out by the circadian clock, and Hattar is careful to explain why it is named the way it is. The word comes from "circa," meaning approximate, and "dien," meaning day. If a person is held under constant conditions with no cues about feeding time, sleep time, or the outside world, a daily rhythm persists, but it is not exactly 24 hours long [s00004]. The average period of the sleep-wake rhythm is 24.2 hours, which means that without sunlight a person drifts about 0.2 hours per day out of alignment with the solar day [s00006]. Sunlight, Hattar explains, is what converts that approximate day into an exact one [s00008].

Hattar insists this small discrepancy is not a curiosity but a survival issue. Doing the arithmetic: a 0.2-hour daily drift accumulates to a full hour in five days and two hours in ten [s00010]. For an animal in the wild, being out of phase with the solar cycle is not merely inconvenient, because "you could either miss food or you could become food" [s00010]. Anticipation of and adjustment to the solar cycle is, in his framing, one of the strongest aspects of survival.

How does the clock get its daily reset? Hattar lays out what is known: in mammals, the eyes are required. People born without eyes, or with damaged optic nerves, cannot adjust to the solar cycle [s00012]. The retina contains rods and cones, which convert photon energy into electrical signals used to build images in the cortex [s00012]. But alongside that familiar system sits a discovery Hattar made together with David Berson and Ignacio Provencio: a small subset of retinal ganglion cells, the cells whose axons leave the retina and project to the brain, are themselves photoreceptors [s00012]. These cells relay information about the light environment subconsciously to the brain region that houses the circadian pacemaker, which in turn entrains all the body's other clocks to the 24-hour light-dark cycle [s00012].