Night
Overview
Night refers to the period of time during which a particular region of Earth, facing away from the Sun due to Earth's rotation, is not directly reached by sunlight and thus grows dark. It is the most fundamental natural cycle that distinguishes the day, and it is deeply involved in astronomical observation, the biological rhythms of living organisms, and human sociocultural activity. The length and timing of night vary greatly depending on latitude and season.
Main Content
Astronomical Definition
Strictly speaking, night refers to the state in which the center of the Sun has descended below the horizon—that is, the time from after sunset to before sunrise. In astronomy, twilight is subdivided according to the Sun's altitude above the horizon into civil twilight (solar altitude 0° to -6°), nautical twilight (-6° to -12°), and astronomical twilight (-12° to -18°). When the Sun descends below 18° beneath the horizon, full astronomical night begins and star observation becomes possible. In the polar regions, phenomena such as the midnight sun or polar night occur, causing night to become short or to last for days at a time.
Earth's Rotation and the Length of Night
Earth rotates once approximately every 23 hours 56 minutes (a sidereal day), while a day measured by the Sun averages 24 hours. Because the axis of rotation is tilted by about 23.5°, the lengths of day and night vary with the seasons. At the spring and autumn equinoxes, day and night are of nearly equal length worldwide, and at the summer solstice the day is longest and the night shortest in the Northern Hemisphere. The higher the latitude, the more extreme the difference in night length across seasons becomes.
The Night Sky and Astronomical Observation
At night, atmospheric scattering decreases, making it a good time to observe celestial bodies such as stars, planets, and galaxies. Most astronomical phenomena—such as the phases of the Moon, meteor showers, auroras, and comets—are observed at night. Light pollution caused by artificial lighting has raised the brightness of the night sky in urban areas, emerging as a problem that greatly reduces the number of observable stars.
Biological Effects
Many organisms have nocturnal life histories, being active at night. Bats, owls, and nocturnal insects are representative examples. Most organisms, including humans, have a circadian rhythm that regulates sleep and wakefulness through melatonin secretion, and night is a key reference point for this rhythm. Excessive exposure to artificial lighting can disrupt the circadian rhythm, potentially causing health problems such as sleep disorders, metabolic diseases, and reduced immunity.
Night in Culture and Art
In human culture, night has been treated as a symbol of fear and romance, reflection and rest. In mythology and religion, night has been depicted with images of darkness, chaos, and death, and has also served as the backdrop for rituals worshipping the Moon and stars. In literature, music, and painting, night frequently appears as a subject evoking solitude, longing, and mystery. Van Gogh's The Starry Night and Beethoven's Moonlight Sonata are representative examples. The nighttime cityscape has become a symbolic image of modern civilization.
The Economy and Society of Night
Since industrialization, the spread of electric lighting has extended the hours of human activity into the night. A so-called "24-hour society" has formed, including night shifts, 24-hour convenience stores, late-night logistics and transportation, and nighttime medical and security services. At the same time, research findings are accumulating that link night work to sleep deprivation, health risks, and an increase in safety accidents. In urban planning, lighting design and noise management have become important tasks.
Latest Trends
In 2024–2025, interest in light pollution and its effects on ecosystems rose significantly. Satellite observations confirmed a trend of increasing brightness and extent of artificial lighting worldwide each year, and countries are moving toward strengthening lighting standards and nighttime lighting regulations. In parts of Europe and the United States, "lights-out at night" campaigns to protect migratory bird routes and wildlife habitats are expanding.
In the field of astronomy, large optical telescopes and space telescopes are expanding the limits of night-sky observation, and the issue of interference by satellite constellations (mega-constellations) on nighttime observation and astrophotography is being discussed as an international issue. In the fields of medicine and sleep science, research is active on the effects of blue light exposure from smart devices and shift work on circadian rhythms, and digital health services that manage individual sleep hygiene are growing. In addition, as part of energy-saving and carbon-neutral policies, there is a growing number of cases in which urban lighting is replaced with LEDs and intelligent lighting control systems are introduced. Meanwhile, nighttime tourism and astrotourism (dark sky tourism) are emerging as new content for local economies.
Related Topics
- [[Day]]
- [[Earth's rotation]]
- [[Astronomy]]
- [[Circadian rhythm]]
- [[Light pollution]]
- [[Moon]]