The beneficial effect of sunlight on all forms of life has been well-known to human cultures worldwide throughout history. However, the importance of darkness for survival, successful reproduction and the overall fitness of all organisms is fully appreciated only by physiologists and environmental biologists. Seasonal variations in environmental conditions (i.e., rainfall, temperature, barometric pressure, food availability) significantly affect reproduction and survival but they are of little predictive value. In contrast, daily fluctuations in light levels and the light spectrum are less dramatic in their impact on life, but were highly predictable throughout evolution. Natural selection has thus favored a strategy of monitoring a day’s length as a predictor of changes in external conditions by the development of the molecular circadian clock, which is sensitive to changes in light/darkness during the day and night. Well-synchronized circadian clockwork ensures that behavioral and physiological processes fluctuate with the daily solar cycle and programs the seasonal changes in physiology via the transduction of the photoperiod into hormonal messages. During the last two decades, energy-efficient lighting technology has shifted from “yellow” high-pressure sodium vapor lamps to new “white” light-emitting diodes (LEDs). As a consequence, nighttime light pollution increased, and the sharp difference between day and night has been erased in many parts of the world, which threatens animal ecology and human health. Studies on humans, laboratory mammals and wildlife suggest that the physiological costs of living under artificial light at night (ALAN) may be due to the disruption of circadian and circannual timing. This overview summarizes the recent findings on the effect of the blurred day/night difference on the circadian clock, nighttime melatonin secretion and photoperiodic changes in mammals and suggests that the gradual decline of fitness due to the increasing ALAN measured in the human population may contribute to the changes in mammalian biodiversity in nature.