Gloom

Gloom is a low level of light which is so dim that there are physiological and psychological effects. Human vision at this level becomes monochrome and has lessened clarity.

Light conditions may be considered gloomy when the level of light in an environment is too low for the proper function of cone cells, and colour vision is lost. In a study by Rothwell and Campbell, light levels described as "gloomy" fell between 28 and 3.6 cd/m⁻².

Low light and lack of colour of this sort may be associated with depression and lethargy. This association was made as far back as the 2nd century by the ancient Greek physician, Aretaeus of Cappadocia, who said, "Lethargics are to be laid in the light and exposed to the rays of the sun, for the disease is gloom." Also, some studies have found weaker electrical activity in the retinas of depressed people, which gave the individuals studied poor visual contrast, meaning that they saw the world in grayer hues. The naturally weak daylight during winter at extreme latitudes can cause seasonal affective disorder, although a percentage of people experience SAD during summer. A solarium or other source of bright light may be used as light therapy to treat winter SAD.