Phototherapy uses specific wavelengths of light to trigger chemical or biological changes in the body. Depending on the condition being treated, it can break down excess bilirubin in a newborn’s blood, kill overactive immune cells in inflamed skin, reset your internal clock, or boost energy production inside your cells. The core principle is the same: light energy is absorbed by a specific molecule in your body, and that absorption sets off a chain reaction that produces a therapeutic effect.
Blue Light and Newborn Jaundice
The most familiar form of phototherapy is the blue light used on newborns with jaundice. Babies with jaundice have too much bilirubin, a yellow pigment produced when red blood cells break down. Normally the liver processes bilirubin so it can be excreted, but a newborn’s liver is often too immature to keep up.
Blue light, typically in the 450 to 495 nanometer range, penetrates the skin and hits bilirubin molecules circulating in the blood near the surface. When a bilirubin molecule absorbs a photon of blue light, its shape physically changes through a process called photoisomerization. The molecule rearranges into a new configuration that dissolves in water, unlike the original form, which does not. The most important of these new forms is lumirubin. Once bilirubin converts to lumirubin, the change is practically irreversible, the molecule is water-soluble, and the body can rapidly excrete it through bile and urine without needing the liver to process it at all. This is why phototherapy can lower bilirubin levels within hours.
UV Light for Skin Conditions
Phototherapy for skin diseases like psoriasis, eczema, and vitiligo works through a completely different mechanism. Instead of reshaping a molecule, ultraviolet light directly targets immune cells that are causing inflammation in the skin.
The most commonly used form is narrow-band UVB, which peaks at 311 to 313 nanometers. When this light penetrates the skin, it damages the DNA of T cells, the immune cells clustered in psoriatic and eczematous lesions. That DNA damage triggers a self-destruct sequence called apoptosis. Research published in The Journal of Experimental Medicine found that 312-nanometer UVB is directly cytotoxic to T cells within psoriatic skin lesions, and that this programmed cell death is likely the primary way phototherapy clears the disease. As the overactive T cells die off, inflammation subsides and the skin begins to heal.
Broader-spectrum UVB (280 to 320 nanometers) and UVA (320 to 400 nanometers) are also used in certain situations, but narrow-band UVB has become the standard because it delivers the therapeutic wavelength more precisely, which reduces unnecessary exposure to wavelengths that only cause sunburn without added benefit.
How Dosing Is Personalized
Your starting dose is based on your individual skin sensitivity. Before beginning a treatment course, a dermatologist may perform what’s called a minimal erythema dose test. Small patches of your skin are exposed to increasing amounts of UV light, and the clinician checks which dose produces the faintest visible redness. Your treatment sessions then start below that threshold and gradually increase. The goal is to deliver enough UV to trigger T cell death without burning the skin. Skin sensitivity is also estimated using the Fitzpatrick scale, a six-point classification from very fair (type I) to very dark (type VI), where fairer skin burns at lower doses.
Bright Light Therapy for Mood and Sleep
Light therapy for seasonal affective disorder, circadian rhythm disruptions, and some forms of depression works through the eyes rather than the skin. Your retinas contain specialized light-sensing cells called intrinsically photosensitive retinal ganglion cells. Unlike the rods and cones you use for vision, these cells contain a protein called melanopsin that responds directly to light and sends signals to brain regions that have nothing to do with seeing images.
One of the primary targets is the suprachiasmatic nucleus, a tiny cluster of neurons that acts as your master body clock. When bright light hits these specialized retinal cells in the morning, the signal resets your circadian rhythm, suppresses melatonin production, and promotes wakefulness. This is why a 10,000-lux light box used for 20 to 30 minutes in the morning can shift the sleep-wake cycle and relieve symptoms of seasonal depression.
The mood effects go beyond just fixing your clock. Studies on patients with seasonal affective disorder have shown that rapidly depleting tryptophan, the building block of serotonin, reverses the antidepressant effects of light therapy. This strongly suggests that light exposure influences serotonin-producing circuits in the brain. Specific pathways have been identified where bright light activates neurons that ultimately quiet a brain region associated with negative emotions and depressive behavior.
Red and Near-Infrared Light Therapy
Red light therapy, also called photobiomodulation, uses wavelengths in the red (around 630 to 670 nanometers) and near-infrared (around 810 to 850 nanometers) range. It works at the cellular level, inside the mitochondria, the energy-producing structures in every cell.
The primary target is an enzyme called cytochrome c oxidase, which sits at the end of your cell’s energy production chain. Under normal conditions, a molecule called nitric oxide can bind to this enzyme and slow it down, reducing your cells’ ability to generate energy. When red or near-infrared light is absorbed by cytochrome c oxidase, it knocks the nitric oxide loose. With that brake released, the enzyme speeds back up, the electrical charge across the mitochondrial membrane increases, and ATP production rises. ATP is the molecule your cells use as fuel for virtually everything they do.
One reason this therapy appears to benefit damaged or stressed tissue more than healthy tissue is that unhealthy cells tend to have higher concentrations of nitric oxide clogging the enzyme. Healthy cells, with less nitric oxide interference, have less room for improvement. The light exposure also generates a small burst of reactive oxygen species inside cells, which at low levels acts as a signaling molecule that activates protective and repair pathways rather than causing harm.
Skin Cancer Risk With UV Phototherapy
Because UV phototherapy involves repeated exposure to ultraviolet radiation, the question of long-term skin cancer risk comes up frequently. The evidence is reassuring for narrow-band UVB, though not without nuance.
A large cohort study of nearly 23,000 Taiwanese psoriasis patients treated with narrow-band UVB found only one case of melanoma and 13 cases of non-melanoma skin cancer. A separate study of 445 patients with darker skin types found zero melanoma cases and no increased risk of non-melanoma skin cancer compared to the general population. A Turkish study comparing 100 treated patients to 100 controls also found no significant increase in skin cancer risk.
The picture is slightly different in lighter-skinned European populations. An Italian study found that the overall rate of non-melanoma skin cancer in patients who received narrow-band UVB was 2.1%, compared to 1.4% in the general European population, a modest but statistically significant increase. Melanoma risk, however, was not elevated. The risk appears to be cumulative: it rises with the total number of treatment sessions and with age.
Home Units vs. Clinical Equipment
Home phototherapy units use the same type of narrow-band UVB bulbs as clinical units, and clinical trials have found them to be comparably effective. The PLUTO trial, a large randomized study published in The BMJ, compared home treatment to outpatient hospital sessions for psoriasis and found no difference in acute side effects like severe redness, blistering, or burning.
The practical differences are in dosing precision and supervision. Hospital units often have built-in sensors that measure light intensity and deliver a precise dose in joules per square centimeter. Many home units lack these sensors and instead prescribe treatment in seconds of exposure time, which means the dose can drift as bulbs age. Safety features on newer home devices include electronic controls that limit the total number of sessions. Once you’ve used your allotted treatments, you need to contact your dermatologist for a new code before the unit will operate again.
Medications That Increase Light Sensitivity
Certain medications make your skin significantly more reactive to UV light, which can lead to severe burns or blistering during phototherapy. The most common culprits are tetracycline antibiotics (including doxycycline and minocycline), retinoids like isotretinoin, some nonsteroidal anti-inflammatory drugs, fluoroquinolone antibiotics, and thiazide diuretics used for blood pressure. Long-term use of several of these medications has also been independently linked to increased skin cancer risk, which compounds the concern when combined with UV exposure. If you’re taking any of these, your phototherapy dose will need to be adjusted or treatment may need to be paused.