By the mid-1800s, artificial light was part of just about everyone’s life. The advent of the light bulb and the burgeoning city nightlife sped up the exchange of ideas and money, but it also affected our bodies and minds in ways that few historians thought to record at the time. Here’s a look at three ways lighting can affect your physiology and why your body might enjoy a break from the bulbs.

Combating Tumors

Melatonin is a hormone that is mostly produced in the absence of light. It is secreted by the pineal gland as part of your natural sleep cycle, and is known to inhibit tumor growth among many other functions. Sleeping with the lights on causes a drop in melatonin and sets the conditions for tumors to spread. Light exposure has been linked to increased breast cancer rates in female night-shift workers, and may be a factor in higher cancer rates in industrialized nations than developing nations.

Regulating Menstrual Cycles

The pineal gland’s preference for darkness also affects female fertility and the menstrual cycle. Some researhcers theorize that moonlight affects melatonin secretion, which in turn affects the delivery of prolactin, a hormone related to lactation and sexual satisfaction. This connection between moonlight and fertility inspired Lunaception, the practice of mimicking moon cycles with sleep-time lighting, as a way to regulate periods and predict ovulation.

Regulating Your Sleep Cycle