# Keep your bedroom dark at night

Published by Pawel Jozefiak. [About this project](https://healthy100.jock.pl/about/), [How we check sources](https://healthy100.jock.pl/methodology/)

- Canonical: https://healthy100.jock.pl/dark-bedroom/
- Language: en
- First published: 2026-09-27
- Evidence checked: 2026-09-28, using AI-assisted source verification
- Clinical review: Not clinically reviewed
- Number 75 of 100 in the list: https://healthy100.jock.pl/#dark-bedroom

Light reaching you while you sleep is linked with worse metabolic health.

**The number:** Brightest nights: 53% higher type 2 diabetes risk than the darkest nights ([The Lancet Regional Health Europe, 2024](https://pmc.ncbi.nlm.nih.gov/articles/PMC11281921/))

## The evidence

- Evidence grade: B. Grade B: consistent large cohort studies, with some trial evidence.
- Basis: Two UK Biobank light sensor cohorts (up to 88,905 adults) and a 20 person laboratory trial, with NIH sleep guidance
- Outcome: type 2 diabetes risk, observational association
- Comparison: brightest versus darkest nights in a UK Biobank cohort
- Impact: 47/100. The impact score is an editorial prioritization heuristic for the general list, not a validated estimate of your personal risk.

## Why it matters

Light can pass through closed eyelids and reach the body clock during sleep. In a UK Biobank study using wrist light sensors, people with the brightest nights had a 53% higher risk of developing type 2 diabetes than people with the darkest nights, and a companion analysis linked brighter nights with higher mortality. A small laboratory study found that one night of ordinary room light during sleep raised heart rate and next morning insulin resistance, which points to a plausible mechanism, though the large studies remain observational.

## How to do it

1. Turn the television and overhead lights off before you sleep rather than leaving them on.
2. Use blackout curtains or a sleep mask if street light reaches your bed.
3. Keep any night light dim, as NIH advises, rather than removing it if you need it.
4. Move charging phones and bright standby lights out of direct view.

## Talk to a clinician first

If you need a light on for safety at night, keep it and choose the dimmest option that works for you.

## Putting it into practice

### Start now

- 5 minutes: Check the room from your sleeping position: identify outside light, screens and bright standby lights, then adjust what you can while keeping safe night lighting.
- 15 minutes: Set up existing curtains or an eye mask for tonight, move bright devices out of direct view and check that any necessary night light still lets you move safely.

### What it looks like

- Turn off the television before sleep rather than leaving it playing in the bedroom.
- Use curtains or an eye mask if outside light reaches your bed.
- Keep a dim night light if you need it to move safely.

### Common mistakes

- Making the room completely dark despite needing light for safety can create a hazard, so keep the lighting you need.
- A phone charging face up can light the room when messages arrive, so turn it face down or move it out of view.

### A structured start

**Set up a darker place to sleep**

1. Stage 1: Notice which light sources reach your sleeping position.
2. Stage 2: Turn off unnecessary screens and room lights; use curtains or an eye mask for outside light.
3. Stage 3: Check comfort and nighttime safety, retaining a dim light wherever needed.

### Trusted how-to guides

- [Healthy Sleep Habits](https://www.nhlbi.nih.gov/health/sleep-deprivation/healthy-sleep-habits), NHLBI, NIH
- [Insomnia](https://www.nhs.uk/conditions/insomnia/), NHS

## Sources

- [Sleep Deprivation and Deficiency: Healthy Sleep Habits](https://www.nhlbi.nih.gov/health/sleep-deprivation/healthy-sleep-habits), NHLBI, NIH, 2022 (Tier 1 guideline)
- [Personal light exposure patterns and incidence of type 2 diabetes](https://pmc.ncbi.nlm.nih.gov/articles/PMC11281921/), The Lancet Regional Health Europe, 2024 (Tier 2 study)
- [Brighter nights and darker days predict higher mortality risk](https://pmc.ncbi.nlm.nih.gov/articles/PMC11513964/), PNAS, 2024 (Tier 2 study)
- [Light exposure during sleep impairs cardiometabolic function](https://pmc.ncbi.nlm.nih.gov/articles/PMC8944904/), PNAS, 2022 (Tier 2 study)

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General health information, not a diagnosis or an individual treatment plan. Personalization changes the order of suggestions; it does not assess your medical risks. Follow the cautions and consult a qualified clinician when needed.

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Reuse: CC BY 4.0, https://creativecommons.org/licenses/by/4.0/