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# One night with the lights on changed how the body handled sugar
- URL: https://the-last-hour.ghost.io/one-night-with-the-lights-on-changed-how-the-body-handled-sugar/
- Published: 2026-10-26T23:00:00.000Z
- Updated: 2026-10-26T23:00:08.000Z
- Description: Heart rate rose, deep sleep thinned, and morning insulin climbed. Melatonin barely moved.
- Author: Vanessa Leikvoll

At 2am, with your eyes closed, your heart is still reacting to the lamp you left on. One night of sleeping in room light was enough to change how the body handled sugar the next morning.

Light reaches the brain through closed eyelids. The brain registers it even while you stay asleep.

In this study, the light switched on the alert side of the nervous system. Heart rate ran higher through the night and heart rate variability ran lower, the pattern you see when the body is on guard.

Sleep got lighter too. The light group spent more time in stage 2 and less in deep slow-wave sleep and REM.

By morning, their bodies needed more insulin to handle the same sugar. The more the alert system had been switched on overnight, the higher the insulin climbed.

Needing more insulin in the morning is the likely link to the mid-morning slump and the pull toward something sweet. The study measured the insulin, not the cravings.

One detail sets this apart. Melatonin levels were about the same in both groups, so this is not the usual story about light and melatonin.

---

The study, published in PNAS in 2022 by sleep researchers at Northwestern University, brought 20 healthy young adults into a sleep lab for two nights. Half slept both nights in near darkness, under 3 lux, and half spent the second night with the overhead lights on at 100 lux.

That is not bright. As the senior author, Phyllis Zee, put it, if you are able to see things really well, it is probably too light.

It was one night, in 20 people, measured in a lab. That is a small study, and it shows a short-term effect, not a long-term outcome.

💡

****Tonight:** Make the room dark before you lie down. TV off, lamps off, and the phone face down or out of sight.  
If you need a light for getting up, use a dim amber or red one placed low, near the floor. If streetlight comes in, use blackout shades or an eye mask, and move the bed so light does not fall on your face.

> One lamp, one night, and both your heart and your insulin noticed. Darkness is the cheapest thing you can give the next eight hours.

This is about light while you sleep. Reading under a low lamp before lights out is a separate question, and not the one this study tested.