How do city lights affect sleep?

Most people now try to sleep under a sky that never truly darkens and whose stars are fading. A 2016 mapping found that over 80 percent of the world's population – and over 99 percent in the US and Europe – live under light-polluted skies, hiding the Milky Way from a third of humanity.1source 1 From volunteers' reports between 2011 and 2022, Kyba and colleagues calculated that the sky is brightening by about 9.6 percent a year.2source 2

Our bodies read light as a "time signal." Evening light, especially blue light, delays the release of melatonin, the hormone preparing us for sleep, and the white LEDs now common in street lighting emit a large share of their light in that blue range. A study of nearly 16,000 Americans found that people in brighter areas tended to go to bed later, sleep less, and feel less satisfied with their rest.3source 3 The link is an association, not proof; the light that reaches a bedroom also depends on curtains and screens. Cities that want darker nights have simple tools: downward-pointing shielded lamps, warmer hues, and dimming in the late hours.