Just a little light goes a long way — in a bad way.
Even small light leaks in your room at night push your body into “day mode.”
In a study from Northwestern University, just one night of sleeping under about 100 lux (think dim room light) raised heart rate, reduced heart-rate variability, and increased next-morning insulin resistance compared to sleeping in near-darkness (under 3 lux). In other words, dim light during sleep keeps your heart running harder than it should — which isn't good for overall recovery.
So it may finally be time to get rid of that nightlight.
How dark is “dark enough”? A consensus guideline published in PLOS Biology recommends keeping the sleep environment as dark as possible — a “can barely see anything in the room” level.
And if you absolutely need a light, make it a very dim, warm, low-kelvin nightlight.
