What Research Says about Light Therapy

The therapeutic light spectrum ranges from red to near-infrared (600 nanometers to 1,100 nanometers), which is at the end of the visible light spectrum and just into the invisible part of the light spectrum. Certain wavelengths clustered in the high-600-nanometer range have been shown to be particularly effective. These wavelengths have good penetration into tissue: They are long enough that they aren’t absorbed by hemoglobin in the blood, and they’re short enough that they’re not absorbed by water. When you’re talking about therapeutic light, both the wavelength and the intensity are very important.

The most robust research has historically been on the use of light for injuries, wound healing, mitigating pain, and dental surgery. Today, there is a lot more clinical activity around the world.

You could pretty much name a health condition and there will be some research on it with light therapy. To name a few, psychiatric disorders such as anxiety and depression seem to benefit from red light therapy (this is different from light boxes that are used to modulate circadian rhythms for people with seasonal affective disorder, which don’t use red light). Light products with red light have been advertised for improving skin and preventing baldness. There is good evidence for the cosmetic applications of light and the rejuvenation of skin at the correct wavelengths.

Research has shown that light can also improve athletic performance and recovery. There’s a huge expanding market of personal light devices for these reasons. What we need to understand better are the mechanisms by which light therapy works and how to track the therapeutic efficacy of treatment.

Light acts as a mild stressor on the body in a dose-dependent manner. We always consider stress to be a bad thing, but at low levels, stress can be good for us. It conditions our tissues and stimulates endogenous protective responses that prime us for either existing or future insult.

There are plenty of other examples of things that are beneficial at low doses and damaging at high doses, such as dietary restriction, exercise, phytotoxins in plants, and ischemic conditioning. With light, we see what’s called a biphasic dose response. At low levels, there are benefits, and if you ramp it up too high in intensity (as with lasers that ablate tissues), then there is damage.

Light serves a broadly protective role, meaning it should be effective against a range of conditions. It’s not a curative agent for any particular disease, but many conditions converge on common downstream pathways, such as oxidative stress, mitochondrial damage, and inflammation. By enhancing the resilience of our cells and our tissues with light, we essentially make them better able to cope with insults.

At a molecular level, there are chromophores (entities that absorb light) that change via chemical reactions when exposed to light at particular wavelengths. As a result, various signaling pathways are triggered. For example, the enzyme cytochrome C oxidase, which lives in mitochondria, absorbs therapeutic light wavelengths and causes a redox change that leads to an increase in ATP production and transient bursts in reactive oxygen species, which can be beneficial at low levels.

We’re still teasing out what the other downstream effects are. Depending on the context, there could be cell proliferation, a release of growth factors or neurotrophic factors in the brain, stimulation of stem cells, and other mechanisms that help ward off disease.

My lab’s focus has been on using light for neurodegenerative conditions. We’ve used mouse models to show that you can protect against features of Parkinson’s disease, Alzheimer’s disease, and retinal degeneration with light applied to the head.

The main problem is penetration—for every millimeter of brain tissue you pass through, you lose about 65 percent of light intensity. In mice, we can successfully penetrate the brain with light because they have thin skulls and there isn’t much brain tissue to get through. In contrast, a human has a thick skull, hair, and lots of overlying tissue—it’s hard to get adequate light to the deeper parts of the human brain affected in conditions such as Parkinson’s disease. Yet studies have still shown that there are improvements in clinical signs and quality of life among Parkinson’s patients when light is directed at their heads. We’re not sure if the light just addresses some of the symptoms of Parkinson’s or if it can actually be neuroprotective and stop cells from dying—that would be amazing news. It’s hard to determine this information from clinical trials right now.

That’s why we’ve started to look into other ways to deliver light. In collaboration with a group in France, we developed a device that is implanted into the midbrain, which is very close to the regions affected in Parkinson’s disease. In animal models, we showed that there were strong reductions in clinical signs of Parkinson’s and that the vulnerable cells were protected using this device. The French team is now pursuing clinical trials.

Another idea we’re working on is whether light needs to be targeted at the head to get a beneficial effect or whether there are also indirect effects. The best study demonstrating this was back in 1989 in Israel, where the researchers showed that treating just one of an animal’s feet with light would induce accelerated healing of wounds on both feet. This suggested there was an indirect effect and possibly a body-wide protective effect of light treatment. We’ve applied this method, which we call remote photobiomodulation, in the context of neuroprotection. When we shielded the head of a mouse with Parkinson’s and treated the whole body, we found that the brain was protected even though it wasn’t directly exposed to the light. In a small pilot study that we are currently replicating, we found that treating the abdomen specifically with light showed strong protective effects on the brain. This could be due to how light affects the microbiome of the gut. This concept of treating the body or other accessible tissues in order to protect the brain might help overcome the penetration issues associated with directly treating the head.

There’s a group in Tasmania that has been developing homemade helmets with LED strips and showing that people who use them have positive effects on quality of life and reducing cognitive symptoms. Right now, the evidence is somewhat anecdotal and based on survey data, but there are some objective measures showing that even with a fairly low-powered light on the skull, people are showing benefits. Keep in mind that with Parkinson’s, there’s a huge placebo effect, and you can’t write that off as a possibility. But most people are being followed up over several years, and these beneficial effects appear to be sustained—their symptoms have improved.

When I first started working on this eight years ago, the idea was really esoteric, and people were skeptical that light could heal the brain. We’ve been working to overcome this skepticism by continuously producing results from robust experiments. But it can be hard to get investment in double-blind randomized controlled trials because there’s no patentable drug at the end of it. People can buy an LED panel online, so there’s not a big financial reward to support such expensive clinical trials.

Recently, there has been more interest in light therapy and knowing how it works and what mechanisms are behind it. In ten years, I believe this field will expand even further purely because there’s nothing you can do to slow or prevent neurodegenerative diseases at the moment. There are no therapies that are neuroprotective. Some drugs might reduce symptoms, but they don’t fix the problem. The beauty of light as a treatment is that it’s simple, safe, and potentially effective. There’s nothing to lose by trying it.

There are so many that I can’t keep up. A few seem to be doing quality scientific testing, while there’s a whole proliferation of others on the market that nobody knows how well they work. As far as I can tell, most devices haven’t been rigorously tested in the context of therapy. But if you can be confident in the intensity of the light and that it’s at the right wavelength, then it should be beneficial.