Night: Definition, Science & Human Experience

Night is the period of ambient darkness caused by Earth's rotation placing a location on the side facing away from the Sun.

Astronomical Definition

• Sunset → Sunrise: Civil night begins at sunset (solar elevation 0°).
• Twilight Phases: Civil (0° to –6°), Nautical (–6° to –12°), Astronomical (–12° to –18°). True night = solar elevation < –18°.
• Duration: Varies by latitude/season. Equator: ~12h year-round. Poles: months of day/night.

Biological & Ecological Role

• Circadian Rhythms: Darkness triggers melatonin (sleep), cortisol decline, body temp drop—essential for health.
• Nocturnal Ecology: 60%+ of animals are nocturnal/crepuscular. Pollination, predation, migration keyed to dark.
• Plant Physiology: CAM photosynthesis (cacti, succulents) fixes CO₂ at night to conserve water.

Human Cultural Construct

• Rest & Sleep: Primary evolutionary niche.
• Social Time: Feasts, rituals, storytelling, nightlife, shift work.
• Symbolism: Mystery, danger, introspection, romance, subconscious (dreams).
• Light Pollution: Artificial light at night (ALAN) disrupts ecosystems, astronomy, health—growing global concern.

Night is not merely absence of light—it is a distinct biophysical regime governing life's rhythms and human imagination.