When crowds fade and routines end, nightfall can quietly change the safety equation in some American cities.

For billions of years, life has depended on Earth's rhythm of day and night. DNA codifies body clocks in all animals and plants, which helps their cells act according to this cycle of light and dark.

BENGALURU, India — If a city is one thing in the daytime, it may well be a different entity altogether come nightfall. For many city-dwellers, the night brings with it a sense of quietude, of rest and ...