Daylight

Daylight is the combination of all direct and indirect sunlight during the daytime. This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by the Earth and terrestrial objects, like landforms and buildings. Sunlight scattered or reflected by objects in outer space (that is, an astronomical object) is generally not considered daylight. Thus, daylight excludes twilight and dawn, even though they are often considered as parts of the diurnal course of the Sun; night time is instead considered as the period in which daylight is not present.

Characteristics

Daylight is present at a particular location, to some degree, whenever the Sun is above the horizon at that location. (This is true for slightly more than 50% of the Earth at any time. The rest is experiencing night.) However, the outdoor illuminance can vary from 120,000 lux for direct sunlight at noon, which may cause eye pain, to less than 5 lux under extreme storm clouds at noon, or under twilight conditions. (Even at its most intense, moonlight measures only about 0.1 lux.)

Daylight and Health

Exposure to daylight is often linked to improved mood, increased productivity, and better health. Lack of daylight can lead to problems like Seasonal Affective Disorder (SAD), a type of depression that's related to changes in seasons. SAD begins and ends at about the same times every year. If you're like most people with SAD, your symptoms start in the fall and continue into the winter months, sapping your energy and making you feel moody.

Daylight in Architecture

In architecture, daylighting is the practice of placing windows, skylights, other openings, and reflective surfaces so that during the day natural light provides effective internal lighting. Particular attention is given to daylighting while designing a building when the aim is to maximize visual comfort or to reduce energy use. Energy savings can be achieved from the reduced use of artificial (electric) lighting or from passive solar heating.

See Also

• Sunlight
• Twilight
• Dawn
• Seasonal Affective Disorder
• Architecture

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