In all animals, melatonin levels decrease during daylight and increase during hours of darkness. That’s because light causes melatonin levels to decline.

Forty years ago, researchers reported that bright lights turned on during darkness will cause a drop in serum melatonin. A report from Northwestern University noted that moderate amounts of light exposure at night harm heart health and increase insulin resistance. The study found that exposure to moderate ambient lighting during nighttime sleep — compared to sleeping in a dimly lit room — harms cardiovascular function during sleep and increases insulin resistance the following morning.

It has been established that light exposure during daytime increases activation of the sympathetic nervous system and can increase heart rate. This study found a similar effect occurred when exposure to light occurred during nighttime sleep.

“Heart rate increases when you sleep in a moderately lit room,” the report reads. “Even though you are asleep, your autonomic nervous system is activated. That’s bad. Usually, your heart rate together with other cardiovascular parameters are lower at night and higher during the day.”

The key takeaway is that you should keep the lights off while you are sleeping. If you need some light to illuminate the floor (to avoid tripping), make sure it is dim.

Amber or red/orange light stimulate the brain less. Avoid using white or blue light.

If there is a lot of outdoor nighttime light where you sleep, wearing an eye mask or blackout shading can be good options.