Researchers had over 11,000 participants wear a light sensor during sleep for one week, and recorded how much light the subjects were exposed to overnight. Then they did two things:
They performed a cardiac MRI on each person 3 years later
They also followed the participants for a full decade, recording how many suffered cardiovascular events such as heart attacks, strokes, atrial fibrillation, congestive heart failure, and cardiac related death.
After adjusting for confounding variables such as obesity, smoking, hypertension, and so forth, they found clear evidence that those who were routinely exposed to 3 Lux or more of light during sleep fared worse. (Lux is a measurement of brightness, or light intensity.) Specifically high light exposure during sleep led to:
Thickening of the heart muscle – a precursor to congestive heart failure – on MRI
An increased clinical risk over the ensuing decade for each of the outcomes mentioned above. In other words, sleeping in a bright room was a risk factor for heart attack, stroke, atrial fibrillation, congestive heart failure, and death.
To give some perspective, 3 Lux is about the light intensity of a very bright full moon at night. By contrast, a typical night light, iPhone screen, or light coming in from another room via the crack in your door, exceeds 3 Lux.
This was an observational study, and as you should know by now if you read this blog regularly, it can therefore be prone to bias and does not definitively prove that light is bad for you. It’s possible, for example, that people who expose themselves to light at night are stressed out, or don’t prioritize sleep, or have some other reason for doing so, and that it’s these factors and not the light that led to cardiac issues. Likewise, perceptive readers will note that light exposure was only measured during one week, which of course means at least some of the participants may have been recorded during a week that was not representative of their normal life.
However, I’m inclined to take these results seriously. For one, the researchers found a “dose-response” curve, meaning that the more light exposure one had, the higher the risk. This type of relationship is highly suggestive of a cause and effect. (To give a famous example that should help reinforce this point: cigarettes and lung cancer also follow a dose-response curve. This means not only that smokers get more lung cancer than non-smokers, but that two pack a day smokers get more lung cancer than one pack a day smokers, who in turn get more lung cancer than “social smokers,” and so forth. It should be obvious that if a cigarette is bad for you, two cigarettes are worse, and so forth. In the same way, in this study those who were exposed to 10 Lux while sleeping did worse than those exposed to 5 Lux, and so forth. If something other than light exposure were the cause of the observed cardiovascular problems, we would not expect to see such a clearly linear relationship between the two).
Moreover, the research fits with other lines of evidence. We know that a dark bedroom tends to lead to better sleep, and we know that good sleep is protective against cardiovascular disease. Therefore it should not surprise us that light exposure during sleep seems to track with cardiovascular health.
There is also an evolutionary angle to take on all of this: until the early 1900s, most humans never saw bright light after dark. It makes sense that our bodies would be “built” to expect darkness when we sleep, and that large amounts of artificial light after dark might be in conflict with our biology.
Finally, I would point out that even if this study somehow got it wrong, there’s really no downside to sleeping in a darker bedroom, so the “risks” of taking steps to reduce your light exposure during sleep are virtually nil.
I would advise readers to take this moment to consider their own light exposure during sleep, and think of ways to reduce it. Buy “blackout” curtains. Turn off lights near your bedroom. Remove night lights, or if you can’t do without them, move them further from your bed. Get electronics like TVs and computers out of your bedroom. Keep your phone in another room when you sleep, or at least turn it off or put it to sleep.
These simple steps might do as much for your cardiac health as exercising or avoiding junk food.
