Melatonin usually serves as a chemical expression of darkness since light exposure suppresses its synthesis. The low level of melatonin is associated with a spectrum of disorders since melatonin is a potent endogenous antioxidant; therefore, individuals should avoid visible light exposure at night. However, evidence also shows that near infrared radiation (NIR), which occupies a major portion of the wavelengths of sunlight, promotes melatonin production and the beneficial effects of sun baths or photobiomodulation therapy may be, at least, partially mediated by the increased local melatonin production induced by NIR. Unlike visible light, NIR can penetrate deep into the human body including the muscle, brain, and even the bones, and its effects on human biology remain to be clarified. Thus, the avoidance of light at night and exposure to sun during the day are equally important to improve melatonin production and human well-being.

Plants Rich with Melatonin
Pistachios are the highest melatonin-containing food that you can find (Video min. 1:24), (660 nanograms melatonin per gram of pistachio)
Mushrooms are not far behind with - 100–400 nanograms per gram. of melatonin content, 
Olive oil and a few other plants are far behind by a factor of 1000 less melatonin content -  71 and 119 picogram/mL, (1 pg = 10⁻¹² grams)