Sleep: Even a low level of light at night may disrupt your blood sugar

Leaving even a dim light on while you sleep may disrupt blood sugar control, a small study in humans suggests

By Clare Wilson

Keeping the TV or a bedside light on overnight could slightly disturb your sleep – enough to disrupt the way our bodies normally keep our blood sugar within a healthy range.

In a small trial, even one night spent sleeping in a room with a light on was enough to give participants slightly worse blood sugar control the next morning.

Previous population studies have found that people who sleep with a light on or the TV on in their bedrooms are more likely to be overweight or have type 2 diabetes. But such research can’t say if it is the light that causes the poor health.

Now, the study by Phyllis Zee at the Northwestern University Feinberg School of Medicine in Chicago and her colleagues supports the idea that the connection is causal, and gives clues about the possible mechanism.

Her team investigated the link by asking 20 healthy volunteers to spend two nights in their sleep lab. On the first night, all participants slept in a very dark room. On the second, half slept with a lighting level of 100 lux, equivalent to keeping a TV or bedside light on or having a bright street light shining through thin curtains.

On both mornings, Zee’s team investigated all the volunteers’ blood sugar control using two common tests involving insulin, the main hormone involved in regulating glucose levels. One measure combined glucose and insulin levels after waking up, and the other involved giving people a dose of glucose and measuring their insulin response.

People who slept in the dimly lit room on their second night had slightly worse blood sugar control than on their first night, when the room was nearly dark. “They thought they slept well, but your brain knows that the lights are on,” says Zee.

People who had two nights under dark conditions had little difference in their blood sugar control.

However, people shouldn’t assume that they need to change their sleeping habits unless the results are borne out in a larger trial, says Jim Horne, who until recently ran a sleep lab at Loughborough University in the UK. “You can’t say that by turning your TV off people are going to lose weight.”

Journal reference: PNAS, DOI: 10.1073/pnas.2113290119