Is it bad to sleep with the TV or a light on? What studies show
Probably not ideal. Even moderate light during sleep measurably raises night-time heart rate and next-morning insulin resistance, and in one large study women who slept with a light or TV on gained more weight.
Sleeping with the television or a light on is probably not doing you any favours, though the evidence is about metabolism and sleep quality rather than anything dramatic. In healthy adults, a single night of moderate room light during sleep measurably raised night-time heart rate and worsened next-morning insulin resistance compared with sleeping in the dark [s1]. And in a large cohort of women, those who slept with a light or a TV on were more likely to gain weight over the following years [s2].
That is a more specific answer than "light is bad for sleep." The point is not that a stray glow will wreck a single night, but that the body reads light during sleep as a signal, and the downstream effects show up in heart rate, blood sugar and, over time, weight [s1][s2].
One night of light changed metabolism
The most direct evidence comes from a 2022 laboratory study published in PNAS. Healthy adults slept one night under moderate light of 100 lux and one night in a dim room of under 3 lux [s1]. The lit night increased night-time heart rate, reduced heart-rate variability — a shift toward sympathetic, "fight or flight" nervous-system activity — and raised insulin resistance the next morning [s1]. The researchers found that the greater the sympathetic shift, the higher the insulin levels, suggesting the nervous-system arousal was part of how light nudged blood-sugar handling in the wrong direction [s1]. One hundred lux is not bright: it is roughly the light of an overcast day indoors, dimmer than a normally lit room, and well within what a television or a hallway light can throw across a bedroom.
The weight signal in a large cohort
The population data come from the Sister Study, which followed 43,722 US women aged 35 to 74 who were not shift workers or daytime sleepers [s2]. Compared with women who slept in the dark, those who slept with a light or television on in the room had a higher risk of gaining 5 kg or more (relative risk 1.17, 95% CI 1.08 to 1.27), of a body-mass-index rise of 10% or more (RR 1.13, 95% CI 1.02 to 1.26), and of developing obesity (RR 1.33, 95% CI 1.13 to 1.57) [s2]. Smaller sources of light — a nightlight, or light from outside the room — were not associated with weight gain, which points to the in-room light or TV specifically [s2]. This is observational and self-reported, it was conducted in women only, and it cannot prove the light caused the weight gain rather than tracking some other habit; but it lines up with what the metabolic experiment would predict [s1][s2].
Why light at night registers at all
Underneath both findings is melatonin, the hormone that signals biological night. A study of 116 healthy adults found that exposure to ordinary room light (under 200 lux) in the hours before bed, compared with dim light under 3 lux, delayed the onset of melatonin in 99.0% of people and shortened the window of melatonin production by about 90 minutes [s3]. Light during the usual hours of sleep suppressed melatonin by more than 50% in most trials [s3]. That is the mechanism a bright screen or an overhead light taps into. For the related question of screens specifically, and why timing may matter more than "blue" light, see the evening-screens evidence; light at night has also been linked to cardiovascular risk.
What it means
The practical upshot is modest and consistent: a darker bedroom is the low-risk default, and falling asleep to the television — which throws changing, sometimes bright light for hours — is the version of this habit with the most evidence against it [s1][s2]. Whether a strictly dark room improves long-term health for any individual has not been shown in a trial, and none of this is medical advice; but there is little cost to turning the set off. For melatonin taken as a supplement rather than the light that governs it, see the melatonin evidence.
Sources
- Light exposure during sleep impairs cardiometabolic function — Proceedings of the National Academy of Sciences, 2022-03-14
- Association of Exposure to Artificial Light at Night While Sleeping With Risk of Obesity in Women — JAMA Internal Medicine, 2019-06-10
- Exposure to Room Light before Bedtime Suppresses Melatonin Onset and Shortens Melatonin Duration in Humans — Journal of Clinical Endocrinology & Metabolism, 2010-12-30
Sources
- Light exposure during sleep impairs cardiometabolic function — Proceedings of the National Academy of Sciences , March 14, 2022
- Association of Exposure to Artificial Light at Night While Sleeping With Risk of Obesity in Women — JAMA Internal Medicine , June 10, 2019
- Exposure to Room Light before Bedtime Suppresses Melatonin Onset and Shortens Melatonin Duration in Humans — Journal of Clinical Endocrinology & Metabolism , December 30, 2010
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