Finsen suffered from Niemann–Pick disease, which inspired him to sunbathe and investigate the effects of light on living things. As a result, Finsen is best known for his theory of phototherapy, in which certain wavelengths of light can have beneficial medical effects.. His most notable writings were Finsen Om Lysets Indvirkninger paa Huden ("On the effects of light on the skin"), published in 1893 and Om Anvendelse i Medicinen af koncentrerede kemiske Lysstraaler ("The use of concentrated chemical light rays in medicine"), published in 1896. The papers were rapidly translated and published in both German and French. In his late work he researched the effects of sodium chloride, observing the results of a low sodium diet, which he published in 1904 as En Ophobning af Salt i Organismen ("An accumulation of salt in the organism").