Darkness, the polar opposite to brightness, is understood as a lack of illumination or an absence of visible light. Human vision is unable to distinguish color in conditions of either high brightness or darkness. In conditions with insufficient light levels, color perception ranges from achromatic to ultimately black. The emotional response to darkness has generated metaphorical usages of the term in many cultures. Referring to a time of day, complete darkness occurs when the Sun is more than 18° below the horizon, without the effects of twilight on the night sky.