Night is defined primarily as the period of darkness characterized by insufficient illumination from the sun, marking a significant boundary in the Earth's diurnal cycle. From a biological perspective, this period governs the regulation of circadian rhythms, which are endogenous biological clocks controlling cycles in behavior, sleep, and metabolism. The transition into darkness triggers changes in hormonal secretion and metabolic rates within many species. Physically, the lack of direct solar radiation alters the atmospheric conditions and the light spectrum available, allowing for the visibility of celestial bodies. The biological processes adapted to this timeframe involve specialized visual systems capable of detecting minimal light levels and the cyclical regulation of sleep-wake patterns, integral to the maintenance of organismal homeostasis.