Conclusion at a glance
Numerous studies over the past 5 years have shown that increased exposure to natural light on a man's testicles and sperm increases sperm motility, or how well individual spermatozoa are able to move and swim. Mobility is the measure of fertility and reproductive health. As a result, many researchers conclude that light therapy can have a significant effect on the treatment of male infertility. [1,2,3,4]
In addition to increased sperm motility, a 2017 published study found Scientific Reports that these treatments are safe and did not cause oxidative DNA damage to the sperm or testicles. 
Optimal hormone functioning already starts in the brain. Different areas of the brain, such as the hypothalamus and pituitary gland, are involved in that process. However, until recently, it was not known that thyroid function also affects the production of steroid hormones.  Thus, improving thyroid function also improves the production of steroid hormones, such as testosterone. To achieve this effect, you must specifically use the red light device on your thyroid gland.
In addition to enhancing steroid action by thyroid hormones, red light therapy can also increase steroid hormones such as testosterone by directly stimulating energy production in associated organs. An example is testosterone production in the testicles. 
Remember, red light can penetrate deep into the body, increasing energy production in specific organs. When the right dose of red light therapy is applied to the testicles, the testosterone level can even rise up to three-fold. 
What qualities make a man a man? That is not an easy question. But from a purely biological perspective, there is a simple answer: testosterone. Men generally produce a lot more testosterone than women, and it is responsible for many of the qualities that people have such as extra body hair, bigger muscles, etc.
Unfortunately, testosterone levels start to drop around your 30th year. This is normal, but that does not make it an easy situation. Declining testosterone levels can pose a host of challenges to a man's life, such as less energy, less sexual drive and performance, less muscle mass, and more weight gain.  To make matters worse, testosterone is adversely affected by a host of common challenges that affect nearly all modern men, such as diet, stress, and sleep deprivation, to name a few. These factors combine to dramatically decrease testosterone levels for many men.
Trying to treat testosterone problems can be a double edged sword. Doctors often recommend taking supplements or medications to reduce the problems of low testosterone or to treat testosterone abnormalities. But these conventional solutions lead to unwanted side effects for many patients. Even more challenging, some men who want to increase their testosterone levels are told by doctors that their value is not low enough to warrant treatment. As a result, dealing with low testosterone can be an extra annoying challenge for many men in their 30s, 40s and 50s.
So it's no surprise that many men want a more natural option to boost their testosterone, and with it their energy, sex drive and physical performance. A promising option is light therapy, because recent medical studies show the immense potential for increasing male testosterone. This has been proven repeatedly in clinical trials on various mammals over the decades, but recent studies on men give researchers and endocrinologists even more cause for optimism.
Peer-reviewed clinical research is always the most reliable indicator of whether a therapy is effective, but individual experiences are also valuable, especially from leading fitness and health professionals. Many of the best in the world have tried light therapy and reported large increases in testosterone, energy, sex drive and performance.
I put a giant red light on my balls to triple my testosterone levels. "I tried red light therapy, a radical (and expensive) alternative treatment, on my testicles - and frankly, I've never felt better”
Medical scientists have been studying the effects of natural light on testosterone production for nearly a century. Research over the past decade, and especially the last few years, is even more illuminating and gives the medical community more concrete reasons to believe in the potential of light therapy for increasing male testosterone.
Previous clinical studies on mammals: While human clinical research in the fields of testosterone and light therapy is still in its infancy and more trials are needed, there's no question that it's promising. For example, the recently published study on men builds on a larger analysis basis for laboratory mammals and their testosterone levels.
A 2013 study in Biomedical Research found that red light therapy at 670 nanometers (nm) increased serum testosterone levels in laboratory rats, with no observed side effects.  Another 2013 study on testosterone in rats published in the Nepal Medical College Journal found that follicle stimulating hormone (FSH) and luteinizing hormone (LH), which are associated with the production of testosterone and natural sex steroids, were elevated in rats subjected to light for 70 days.  The FSH and LH pathway is essentially the same in humans, and they are called "gonadotropins" because of their ability to stimulate testicular function.
Other studies of dog and boar sperm motility have shown the same type of positive effect. [11,12]
Italian pilot study: A 2016 randomized, placebo-controlled pilot study conducted by the University of Siena in Italy assessed 38 men with diagnosed low sexual desire. Researchers measured their testosterone levels and divided the men into two groups, with one group receiving a clinical dose of light therapy in the early mornings. In addition to the higher sexual satisfaction, the men in the light-treated group saw their T values increase significantly. The control group did not see an increase in testosterone, but the active light therapy group showed a massive increase from about 2,1 ng / ml to 3,6 ng / ml in just 2 weeks. 
Dr. Andrea Fagiolini, lead investigator of the study, explained the findings, "The elevated testosterone levels explain the greater reported sexual satisfaction." In the Northern Hemisphere, the body's testosterone production naturally declines from November to April and then rises steadily through the spring and summer, peaking in October. You can see the effect of this in the reproduction ratios, with the month of June having the highest degree of conception. "Using the [light therapy device] really mimics the natural effect." 
Although more male testosterone has been found with light therapy in numerous studies, the exact reasons and biological mechanisms of action are not fully understood at this time. In other articles we have explained how the human body absorbs red and shortwave infrared light, which promotes an increase in cellular chemical energy transport - also known as adenosine triphosphate (ATP). Just as improved function can be seen in various body processes through increased ATP energy, many of these studies suggest that red and infrared light wavelengths can stimulate the Leydig cells, which are responsible for producing testosterone in the testicles. It is theorized that this increased energy production can naturally help increase testosterone hormone levels. [3,9,10]
Leading researchers have posited a number of other theories and are not mutually exclusive. Some believe that red and short-wave infrared wavelengths stimulate photoreceptive proteins in the testes, resulting in higher testosterone production, as shown in studies as early as 1939. 
Researchers from the above 2016 University of Siena study suggested that light therapy interacts with the pineal gland in the brain, which plays an important role in human reproduction. 
While medical science is still trying to identify the exact mechanisms, it is quite clear that there is a relationship between natural light and male testosterone in both humans and other mammals. This ties in with the vast amount of anecdotal evidence we hear all the time from trainers, athletes, doctors, and health and fitness professionals reporting large testosterone increases after using large red light therapy lamps.
No one wants to endanger their sexual health with risky or unproven treatment. But some men may have reported negative effects on their testes from using some forms of energy therapy in the past. The key, however, is that those negative effects come from sub-optimal products with too long wavelengths of light that produce too much heat.
Emerging research on light therapy and its effect on male testosterone, fertility and sex drive is extremely encouraging. Together with the previous laboratory research and the current experiences of top athletes and trainers that this therapy use, it is clear that light therapy has the potential to become a game-changer for men looking to increase natural testosterone production.
A few important points from this article are:
- In most cases, red light treatment directly on the testicles for short regular periods would increase testosterone levels over time.
- Stick to LED light therapy for maximum safety and benefits. Visible red (600-700nm) LEDs are optimal. Session time of 2-20 minutes depending on the light intensity / heat.
- Red or infrared light from an LED source (600-950nm) can be safely applied to the testes with no risk of side effects or harm, while delivering all the benefits as described above (improved fertility, improved testosterone production, etc.).
- Sunlight can also be used on the testes, but only for short periods and it is not without risks.
- Avoid exposure to blue / UV light.
- Avoid any kind of heat lamp / light bulb.
Scientific sources and medical references: