Using Light (Sunlight, Blue Light & Red Light) to Optimize Health

KEYPOINTS

• 01

  Light shapes the body through two routes: eyes and skin

  Huberman explains that wavelength determines what light does. Short wavelengths such as UVB and blue strongly drive melanopsin-containing retinal ganglion cells. Long wavelengths such as red and near-infrared penetrate deeper into skin and reach cellular structures like mitochondria. Nature rarely relies on a single mechanism, so many effects of light, including on hormones, pain and mood, run through both the eyes and the skin.

• 02

  Melatonin translates day length into a body-wide signal

  Light hitting the eyes signals the pineal gland, deep in the brain, to stop releasing melatonin. Long summer days therefore mean a shorter nightly melatonin signal, and winter a longer one. Endogenous melatonin has regulatory effects: it promotes bone-building osteoblasts, suppresses gonad maturation (one reason high childhood levels delay puberty), affects placental development, and regulates sleepiness. It also has protective effects as an antioxidant and immune activator. Huberman stresses that the benefit comes from its daily and seasonal rise and fall, not from simply having more.

• 03

  Protect your nighttime melatonin

  Bright overhead light in the middle of the night drives melatonin to near zero almost immediately. If you get up, use the minimum light needed to move safely, ideally dim amber or red light, which barely affects the melanopsin cells unless very bright. Shift workers should keep their sleep period dark too. Huberman is cautious about melatonin supplements, saying most doses are supraphysiological and flatten the natural seasonal rhythm. Anyone pregnant should talk to their doctor before taking it.

• 04

  Sun on the skin raised sex hormones in a 2021 study

  Huberman reviews a 2021 Cell Reports paper, "Skin exposure to UVB light induces a skin-brain-gonad axis and sexual behavior." In mice, UVB raised testosterone and estrogen, increased mating and gonad size, and improved follicle maturation. In humans, after a few days of covering up to set a baseline, UVB exposure raised estradiol, progesterone and testosterone in both sexes and increased romantic passion, sometimes after just one or two sessions. Measured across a year, testosterone was lowest in winter and highest from June to September. Testosterone rose more in men from countries with low UV exposure, which parallels darker skin needing more sun for vitamin D3.

• 05

  Bright light reduces pain through the eyes and skin

  UVB on the skin increases beta-endorphin release. A recent Neuron paper found that light captured by melanopsin cells in the eye signals the ventral lateral geniculate and intergeniculate leaflet, which drive the periaqueductal gray to release endogenous opioids. This helps explain why people tolerate physical and emotional pain better on long, bright days.

• 06

  UVB to the eyes primes immune defenses

  Sufficient UVB to the eyes activates a sympathetic nervous system pathway that makes the spleen deploy immune cells and molecules. Huberman argues that fewer summer colds reflect better defenses as well as warmer weather. Wound healing is faster with more UVB, and a PNAS study showed that light on melanopsin cells helps drive stem-cell turnover in skin, hair and nails, which grow faster on longer days.

• 07

  The right light at the wrong time worsens mood

  Citing Samer Hattar of the National Institute of Mental Health, Huberman recommends getting as much light in your eyes and on your skin early and throughout the day as is safely possible. But a melanopsin pathway to the perihabenular nucleus bypasses the body clock, and activating it at night reduces dopamine and can promote low mood. He advises avoiding artificial bright or UV-rich light from roughly 10 p.m. to 4 a.m., and from 8 p.m. if you're prone to depression.

• 08

  Even dim light during sleep affects the heart and metabolism

  In a study he describes, healthy adults slept a full night in either under 3 lux or about 100 lux of overhead light. Melatonin was unaffected in both groups. But the 100-lux sleepers had higher nighttime heart rate, lower heart rate variability and greater insulin resistance the next morning. He suggests a free light-meter phone app to gauge your bedroom's brightness.

• 09

  Red light reaches mitochondria

  Red and near-infrared light can enter cells and boost mitochondrial ATP production while lowering reactive oxygen species, which build up with age. That's the mechanism behind evidence-backed low-level light therapy for acne, scarring, wound healing and pigmentation, which is usually applied to targeted patches of skin. Huberman notes there are few data on whole-body red light and that most infrared saunas don't get hot enough to produce sauna's established effects.