Daylight or the light of day is the combination of all direct and indirect sunlight outdoors exclusively during the daytime (and perhaps twilight). This includes direct sunlight, diffuse sky radiation, and (often) any/all of the above reflected from the Earth and terrestrial objects. Daytime is the period of time each day when daylight occurs. While perceived moonlight is in fact light from the sun reflected towards the Earth, and is hence to be considered "indirect sunlight", it is not considered daylight as it occurs outside of the hours that one would consider "daytime".

Daylight is present, to some degree, whenever the sun is up, but the outdoor illuminance can vary from 100,000 lux for direct sunlight at noon, which may cause eye discomfort, to less than 5 lux for the thickest storm clouds with the sun at the horizon, which may make shadows from distant streetlights visible. It may be darker under unusual circumstances such as a solar eclipse or very high levels of atmospheric smoke.

Daylight is widely accepted to have a positive psychological effect on the human being, and consequently more cases of mental health problems are registered during the winter months than during the summer months due to the shortened periods of daylight. Cases of depression specifically linked to limited daylight are referred to as seasonal affective disorder.

Daylighting is lighting, such as windows and skylights whose light is really daylight. This type of lighting is chosen to save energy, to avoid adverse health effects of over-illumination by artificial light and also for decoration.

In recent years, work has taken place to recreate the effects of daylight artificially. This is however expensive in terms of both equipment and energy consumption and is applied almost exclusively in specialist areas such as movie-making, where light of such intensity is required on a professional level.

See also

• Twilight
• Moonlight

Light | atmospheric and ocean optics

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