A new study published in the Proceedings of the National Academy of Sciences has shown that light exposure while sleeping impairs cardiometabolic function. The study included 20 healthy adults who were randomized to sleep in either a moderately lit room or a dimly lit room. After only one night in the moderately lit room, participants showed an increased nighttime heart rate, decreased heart rate variability, and increased insulin resistance.

In essence, higher heart rate variability is a measure of your body’s resilience and adaptability to change. Decreased heart rate variability, in combination with increased heart rate, indicates that the nervous systems of participants in the moderately lit rooms were more active than they should have been. This likely leads to less restful sleep, which could add up quickly for those who consistently sleep in moderately lit rooms.

Insulin resistance occurs when muscle, fat, and liver cells don’t respond effectively to insulin. Insulin is a hormone that helps control the amount of glucose (sugar) in the bloodstream, so insulin resistance can lead to increased blood sugar. Over time, this can lead to type 2 diabetes and weight gain.

One scientist suggested that if you can easily see around your bedroom at night, it is probably too light.

Sources: PNAS, Sleep Foundation, Cleveland Clinic, Clinical Advisor, Science Daily