Blue light is a light spectrum, emitted by the sun. It also emitted by technology devices and therefore has fallen into disrepute. First of all, blue light in the evening (typical with TV/Computer/Cellular phones) is not healthful. It is especially relevant that it inhibits melatonin, which is needed for sound sleep.  Hence, we are warned about the deleterious effects of using our phone, tablets, TVs and computer late at night.
Is blue light both good and bad?
In addition, melatonin has anti-cancer properties. However, while blue light exposure reduces evening melatonin (bad), it also inhibits melatonin production during the day (good). Why? Because melatonin lets the body relax and sleep. That is the last thing we want by day, because we should be alert and working. Blue light, while reducing melatonin at night, is wonderful by day. Another positive effect is its ability to call T-cells (important immune-system cells) into play. And of course, T-cells are soldiers that can destroy noxious, invading microorganisms.
What more can blue light do?
In addition, full–body exposure to blue light decreases blood pressure, lessens arterial stiffness, and improves endothelial function.
Another attribute is its ability to cause subdermal fat tissue to decrease in size. In other words, it can cause fat loss. Thus, the action of sunlight may help one to stay slim or become slim. The researchers showed that daily exposure of fat cells to blue light resulted in decreased lipid droplet size and increased fat breakdown rate. The researchers had been doing research on light and diabetes, and they serendipitously found that the light could be an asset in maintaining (or producing) a slim body.
Set your circadian rhythm.
Blue light also helps to reset the circadian rhythm of the body. In conclusion, remember that blue light, which is particularly beneficial during the daytime, is disruptive at night and inhibits melatonin, needed for sleep. Therefore, get out in the early morning and get your share of blue light throughout the day. And when it becomes dark, go to bed early and avoid blue light. Avoid sunburn when you are soaking up the sun. Simply cover up when you have had enough, and avoid sunscreens, which have been shown to actually increase sunburning.
To learn all the facts about the healthful effects of sunlight, read my book, Embrace the Sun, available at Amazon. https://www.amazon.com/Embrace-Sun-Marc-B-Sorenson/dp/069207600X
 Vandewalle G, Collignon O, Hull J.T, Daneault V, Albouy G, Lepore F, et al. Blue light stimulates cognitive brain activity in visually blind individuals. J Cogn Neurosci. 2013 Dec;25(12):2072.
 Knufinke M, Fittkau-Koch L, Møst EIS, Kompier MAJ, Nieuwenhuys A. Restricting short-wavelength light in the evening to improve sleep in recreational athletes – A pilot study. Eur J Sport Sci. 2018 Nov 14:1-8. [Epub ahead of print].
 de Almeida Chuffa LG, Seiva FRF, Cucielo MS, Silveira HS, Reiter RJ, Lupi LA. Mitochondrial functions and melatonin: a tour of the reproductive cancers. Cell Mol Life Sci. 2018 Nov 14 [Epub ahead of print].
 Phan TX, Jaruga B, Pingle SC, Bandyopadhyay BC, Ahern GP. Intrinsic Photosensitivity Enhances Motility of T Lymphocytes. Sci Rep. 2016 Dec 20;6:39479.
 Stern M, Broja M, Sansone R, Gröne M, Skene SS, Liebmann J. et al.
 Ondrusova K, Fatehi M, Barr A, Czarnecka Z, Long W, et al.. Subcutaneous white adipocytes express a light sensitive signaling pathway mediated via a melanopsin/TRPC channel axis. Scientific Reports 2017 November 27;7:16332.
 Bonmati-Carrion MA, Arguelles-Prieto R, Martinez-Madrid MJ, Reiter R, Hardeland R, Rol MA, Madrid JA. Protecting the melatonin rhythm through circadian healthy light exposure. Int J Mol Sci. 2014 Dec 17;15(12):23448-500.