Give yourself to the dark side. It is the only way you can save your friends. –Darth Vader
Many of us know to stash cannabis wares away from light. An opaque mason jar in the back of a closed cabinet is a classic storage solution. That begs the question — how does light affect post-cultivation cannabis? And if light is so bad, why do dispensaries blare stadium lights over their product displays?
First, the basics: light is a form of energy, namely electromagnetic radiation that occurs at certain wavelengths. Visible (optical) light has wavelengths that run from roughly 400 nanometers (violet) to 700 nanometers (red). Ultraviolet light (or UV radiation) has shorter wavelengths (300-400 nanometers), and infrared light has longer wavelengths (700-800 nanometers). Shorter wavelengths are more energetic and potentially harmful, which is why UV rays damage our skin (i.e., sunburn).
[Graphic 2/ Source: Philip Ronan, Wikimedia Commons]
Cannabis cultivation requires a lot of light. Even UV-B radiation, with very short wavelengths of 280-315 nanometers, may benefit live cannabis plants in controlled doses.  Live plants utilize light to power growth and metabolism.  But once cannabis has been harvested, everything changes. As early as 1976, researchers identified light exposure as one of the greatest causes of cannabinoid loss in stored cannabis. 
Radiation degrades organic compounds. Visible light accelerates molecular movement (i.e., heat). Ultraviolet light, on the other hand, ionizes molecules, adding or removing electrons and thereby breaking molecular bonds. We might say that light shakes things up. In our case, the organic matter from harvested cannabis absorbs energy from light. This energy excites electrons and may directly alter the chemical structure of a sample, facilitating degradation.[Graphic 3/ Source: Rice University, CC 4.0]
If this sounds a little crazy, consider that museum exhibits often prohibit flash photography and control light schemes with the utmost precision. Per the National Gallery of Art, “Damage occurs because light is radiant energy… Low light levels over a long period of time can cause equal or even greater degradation as intense light for a short period.”
In cannabis, Δ 9-tetrahydrocannabinol (THC) naturally degrades to cannabinol (CBN) over time. Light exposure supplies energy and speeds up this process. The ratio of THC to CBN in a stored sample of cannabis can actually be used to indicate age and quality of storage. [4, 5]
Lindholst (2010) examined tetrahydrocannabinolic acid (THCa) in cannabis extracts. Samples exposed to daylight degraded at a half-life of 35 days, while those kept in darkness degraded at a half-life of 91 days (an approximate 250% difference).  Significant degradation of cannabidiol (CBD) has also been noted in cannabis oil stored under laboratory lighting; CBD may transform first to THC prior to degradation to CBN. 
It’s fair to say that sunlight must be avoided. The ionizing properties of UV radiation make it particularly powerful against cannabis preservation. But many conventional light sources used in homes and businesses also emit UV radiation, albeit at less significant levels compared to sunlight.  Furthermore, the ability of visible (and infrared) light to transfer energy should not be ignored given that heat accelerates cannabis degradation.  Going back to the museum connection, the National Gallery of Art considers infrared light — with its longer wavelengths — a “common source of radiant destruction.”
Dispensaries display cannabis under bright lights because that makes products more attractive. The consumer usually wants to see what they are getting. But if you want to keep cannabis products fresh and potent, embrace the power of the dark side. Store cannabis products in a cool, dark spot, and they will last much longer.
- Lydon J, et al. “UV-B Radiation Effects on Photoynthesis, Growth and Cannabinoid Production of Two Cannabis Sativa Chemotypes.”Photochemistry and Photobiology. 1987, vol. 46, no. 2, pp. 201-206. Journal Impact Factor = 2.214, Times Cited = 5 (PubMed)
- Bilodeau, Samuel Eichhorn, et al. “An Update on Plant Photobiology and Implications for Cannabis Production.” Frontiers in Plant Science, vol. 10, 2019, doi:10.3389/fpls.2019.00296. Journal Impact Factor = 4.298, Times Cited = 1 (PubMed)
- Fairbairn, J. W., et al. “The Stability of Cannabis and Its Preparations on Storage.” Journal of Pharmacy and Pharmacology, vol. 28, no. 1, 1976, pp. 1–7. doi:10.1111/j.2042-7158.1976.tb04014.x. Journal Impact Factor = 2.405, Times Cited = 10 (PubMed)
- Ross, S.A., and ElSohly, M.A. “CBN and D9-THC Concentration Ratio as an Indicator of the Age of Stored Marijuana Samples.” United Nations Office on Drugs & Crime, 1999.
- Trofin, Irenne, et al. “The Influence of Long-Term Storage Conditions on the Stability of Cannabinoids Derived from Cannabis Resin.” Revista de Chimie, vol. 63, Apr. 2012, pp. 422–27. Journal Impact Factor = 1.605, Times Cited = 11 (ResearchGate)
- Lindholst, Christian. “Long Term Stability of Cannabis Resin and Cannabis Extracts.” Australian Journal of Forensic Sciences, vol. 42, no. 3, 2010, pp. 181–190., doi:10.1080/00450610903258144. Journal Impact Factor = 0.99, Times Cited = 13 (ResearchGate)
- Trofin, Irenne, et al. “Long-Term Storage and Cannabis Oil Stability.” Revista Da Chimie, vol. 53, Mar. 2012, p. 294. Journal Impact Factor = 1.605, Times Cited = 6 (ResearchGate)
- Klein, Rachel S., et al. “The Risk of Ultraviolet Radiation Exposure from Indoor Lamps in Lupus Erythematosus.” Autoimmunity Reviews, vol. 8, no. 4, 2009, pp. 320–324., doi:10.1016/j.autrev.2008.10.003. Journal Impact Factor = 8.961, Times Cited = 4 (PubMed)
- Wang, Mei, et al. “Decarboxylation Study of Acidic Cannabinoids: A Novel Approach Using Ultra-High-Performance Supercritical Fluid Chromatography/Photodiode Array-Mass Spectrometry.” Cannabis and Cannabinoid Research, vol. 1, no. 1, 2016, pp. 262–271., doi:10.1089/can.2016.0020. Times Cited = 5 (PubMed)