The promising evidence of red and infrared light therapy for increasing testosterone in men
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 for 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 foundScientific Reports that these treatments are safe and did not cause oxidative DNA damage to the sperm or testes. 
Optimal hormone functioning already starts in the brain. Different brain regions, 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 through 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 that 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, testosterone levels can actually increase up to threefold. 
The testosterone problem in men
What qualities make a man a man? That's not a simple question. But from a purely biological perspective, there is a simple answer: testosterone. Men generally produce much more testosterone than women, and it is responsible for many of the qualities that people see as extra body hair, bigger muscles, etc.
Unfortunately, testosterone levels start to drop around the age of 30. This is normal, but that doesn't make it an easy situation. Declining testosterone levels can bring 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 lower testosterone levels for many men.
Regular and alternative treatments
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 levels are 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 demonstrate its immense potential for increasing male testosterone. This has been repeatedly proven in clinical trials on various mammals over the past few decades, but recent studies on males give researchers and endocrinologists even more reason for optimism.
Peer-reviewed clinical research is always the most reliable indicator of whether a therapy is effective, but individual experiences are also a valuable source of information, especially from leading fitness and health professionals. Many of the best in the world have tried light therapy and reported big 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 honestly, I've never felt better"
Ben greenfield in: Men's Health
Clinical research shows the potential for increasing testosterone with light therapy
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 enlightening and gives the medical community more concrete reasons to believe in the potential of light therapy for increasing male testosterone.
Previous clinical research on mammals: Although clinical research in humans on testosterone and light therapy is still in its infancy and more trials are needed, there is no doubt that it shows promise. For example, the recently published research on men builds on a larger base of analysis for laboratory mammals and their testosterone levels.
A 2013 study inBiomedical Research found that red light therapy at 670 nanometers (nm) increased serum testosterone levels of lab rats, with no observed side effects.  Another 2013 study on testosterone in rats published in theNepal Medical College Journal found that follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which are related to the production of testosterone and natural sex steroids, were elevated in rats subjected to light for 70 days.  The FSH and LH pathway are essentially the same in humans, and they are referred to as "gonadotropins" for their ability to stimulate testicular function.
Other studies on dog and boar sperm motility have shown the same type of positive effect. [11,12]
Italianpilot 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 rise significantly.The control group saw no 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, the study's principal investigator, explained the findings: "The increased testosterone levels explain the greater reported sexual satisfaction." In the Northern Hemisphere, the body's testosterone production naturally declines from November to April, then rises steadily through spring and summer with a peak in October. You can see the effect of this in the reproduction rates, with the month of June having the highest conception rate. "Using the [light therapy device] really mimics the natural effect." 
How does red light therapy increase testosterone levels?
Although numerous studies have found increased male testosterone with light therapy, the exact reasons and biological mechanisms of action are not fully understood at this time. In other articles, we've explained how the human body absorbs red and short-wave infrared light, which promotes an increase in cellular chemical energy transport - also known as adenosine triphosphate (ATP). Just as improved function can be observed 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 may naturally help increase the testosterone hormone.[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 evidenced by studies as far back as 1939. 
Researchers from the aforementioned 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, both in humans and other mammals. This ties in with the large body of anecdotal evidence we hear all the time from trainers, athletes, doctors, and health and fitness professionals who report large testosterone increases after using large red light therapy lights.
An important note about warmth and safety
No one wants to jeopardize their sexual health with a risky or unproven treatment. But some men may have reported negative effects on their testes when using some forms of energy therapy in the past. The key, however, is that those negative effects stem from suboptimal products with too long wavelengths of light that produce too much heat.
Conclusion: Light therapy can be a game-changer for boosting natural male testosterone
Emerging research on light therapy and its effect on male testosterone, fertility and sex drive is extremely encouraging. Together with the previous lab research and the current experiences of top athletes and trainers using this therapy, it is clear that light therapy has the potential to be a game-changer for men seeking an increase in natural testosterone production.
A few key points from this article are:
- In most cases, a red light treatment directly on the testicles for short regular periods should 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 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 damage, while delivering all of the benefits 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/bulb.
Scientific sources and medical references: