Can leaving the light or TV on at night make you gain weight? Shining light on the subject

This week in JAMA Internal Medicine, researchers published an article analyzing the relationship between artificial light at night (which they call ALAN– hi, Alan!) and increased risk of weight gain among women (they used a large cohort of women for the study). They say that exposure to ALAN does increase the risk of weight gain, and “further prospective and interventional studies could help elucidate this association and clarify whether reducing exposure to ALAN can promote obesity prevention.”

In other words:

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As you can imagine (or even saw), the popular press was all over this.

• Sleeping with the TV on may make you gain weight
• Sleeping with lights on and weight gain in women tied in new study
• And many many more…

What should we make of this? Before reading the study (yes, I read through the original, so you don’t have to), I thought, “of course they’ll find an association between ALAN and weight gain. Light exposure disturbs and disrupts sleep, reduces quality and duration, etc. We know all this can contribute to weight gain.”

But, it turns out that their results are (as usual, in science) really complicated. Here are some of the complex results:

• The association between ALAN and increased risk of weight gain was stronger for women with lower BMIs (< 25 and <30) than women with higher BMIs (>30);
• The association between ALAN and increased risk of weight gain was stronger for women who ate healthier diets and for women with increased leisure-time physical activity;
• Women who slept with no light in the room, who ate a less healthy diet and/or who did less leisure-time physical activity had increased risk for weight gain;

So women who weighed less or who ate more so-called healthy diets or who were more physically active were more at risk for weight gain from light sources while sleeping than those who weighed more, ate less so-called healthy diets, or who were less physically active. Could this be because the added factor of light pales, as it were, in comparison to other potential factors? Reading the details did not shed much light for me here. The researchers also didn’t offer an explanation.

Here’s a less surprising result from the article:

Women with greater exposure to ALAN had higher mean BMI… and were more likely to be non-Hispanic black. They were less likely to have consistent waking and bedtime patterns and more likely to have less sleep, take a longer time to fall asleep, wake up at night, and take naps. They also used less sleep medication.

How we sleep and how we eat are related in a bunch of ways. How we sleep and how our bodies respond are also related in a bunch of ways, which include socio-economic, geographic and other external features outside of individual eating and activity behaviors. Women who sleep less, have less control over their sleep schedules, have regular sleep disruption, but don’t take sleep medication are bound to experience increased stress and other behavioral effects. And we know that black women, according to many sources, are more likely to sleep less and also suffer from sleep disorders (like sleep apnea), even controlling for BMI.

The researchers know this, too, and admitted as much (although they focused on individual factors rather than the social determinants of health, which I think diminishes their analysis):

We were unable to disentangle the temporal relationship between exposure to ALAN and other factors, including unhealthy diet, sedentary lifestyle, stress, and other sleep characteristics. Thus, we cannot exclude the possibility that the association between ALAN and obesity is not causal, despite multivariable adjustment and various sensitivity analyses.

So no, ALAN is not good for us (sorry, Alan!). But the reasons why we sleep with light vary a lot, depending on the constraints and realities of our lives. We may have control over some of these factors, and no control at all over others.

My main takeaway is this: sleeping in darkness is a necessity, but for many of us it’s a luxury we can’t have as often as we might need it. Here’s to blissful darkness for everyone.

Readers, do you sleep in darkness? Do you use a sleep mask? I do. Do you want light in your room when you sleep? Do you have to deal with light sources that you’d rather not? Do light sources provide comfort or company? I’m curious about what your habits are.