Essentials: Using Light to Optimize Health

This Huberman Lab Essentials episode, presented by Andrew Huberman, a professor of neurobiology and ophthalmology at Stanford School of Medicine, starts from one claim. Light affects so much of our biology because the body can convert it into electrical signals, hormone signals and cascades of biological pathways. The episode says those cascades can change which genes cells express, and that this holds throughout the lifespan. Light is described as electromagnetic energy that can cause reactions in cells. Ripening fruit is the everyday example: unripe fruit that gets a lot of sunlight ripens because that energy acts on the plant, the tree or the fruit itself.

Two more physical principles follow. First, light comes in many wavelengths, illustrated by the prism on a Pink Floyd album cover that splits white light into reds, oranges, greens, blues and purples. Second, different wavelengths penetrate tissue to different depths because of how their waves travel. The episode then says every biological function of light depends on whether a cell or cell compartment absorbs it, reflects it, or lets it pass through.

Three routes by which environmental light becomes a biological event: - The eyes: photoreceptors at the back of the eye, in two main types: rods, which are elongated, and cones, which look like little triangles. - The skin: the epidermis holds keratinocytes and melanocytes. With light exposure, melanocytes switch on genetic and other programs that increase pigmentation, which we call tanning. - Every other cell: if light reaches a cell in bone, bone marrow, heart, liver or spleen, it changes how that cell works, "for better or for worse."

The third route comes with a qualification. Light never reaches many organs that sit deep under the skin. The spleen is the example: light will never land on it directly, yet it still responds to light through indirect pathways. The relay starts in the eye, at the intrinsically photosensitive ganglion cell, also called the melanopsin cell. It contains a photopigment that absorbs short-wavelength light arriving in sunlight.

The melanopsin cells connect to brain stations that in turn connect to the pineal gland, which is described as a pea-sized gland in the middle of the brain that releases melatonin. Light activates the melanopsin cells, and that shuts down melatonin production. Melatonin therefore acts as a "transducer": it reports how much light, on average, is in your surroundings. For people in the northern hemisphere, this means more melatonin is released in winter than in summer. The episode calls this a hormone-based calendar that uses light to track where you are on the journey around the sun.

what it means is that the environment around us is converted into a signal that changes the environment within us.

The episode separates this natural, endogenous melatonin from supplements. The host says they are not a big fan of melatonin supplementation for a number of reasons. As an aside, they say most supplements contain levels "far too high to really be considered physiological" and in most cases supraphysiological, and that melatonin has effects beyond sleep. The rest of the discussion is about the melatonin the body makes according to the time of year.