Sitting by a window may improve blood sugar levels for type 2 diabetes

Many of us sit by a window to boost our mood, but exposure to natural light during the day also seems to improve blood sugar control in people with type 2 diabetes.

Our cells and tissues follow circadian rhythms, 24-hour cycles of metabolic activity that regulate things like blood sugar levels. Studies have shown that exposure to artificial light at night disrupts these rhythms, raising blood sugar levels, while spending more time outside in sunlight seems to enhance the body’s response to insulin, a hormone that helps keep these levels in check.

But none of these studies explored the potential benefits of being exposed to natural light through a window, even though most people spend the vast majority of their time indoors, says Joris Hoeks at Maastricht University in the Netherlands.

To learn more, Hoeks and his colleagues recruited 13 people with type 2 diabetes with an average age of 70, who spent 4.5 days in a room where they were solely exposed to natural light, through large windows, between 8am and 5pm.

The participants, who continued to take their usual diabetes medications, mainly sat at a desk where they had access to their phones and computers, with these screens set to a low level of brightness. In the evenings, they were exposed to dim artificial light and had access to their devices until 11pm, before sleeping in complete darkness until 7am. They all ate three similar meals a day, designed not to make them gain or lose weight, and did the same exercises at fixed points across the 4.5 days.

The researchers also carried out a very similar experiment with the same participants, but this time they sat in windowless rooms with only artificial lighting. This was done either a month before or a month after the natural-light part of the study.

During both experiments, the participants wore devices that continuously monitored their blood sugar levels, although technical problems meant this data was only available for 10 of them.

This revealed that, during the natural-light week, the participants’ blood sugar levels remained within a healthy range 50 per cent of the time. In the artificial light experiment, this range was only achieved 43 per cent of time.

Definitions of a healthy blood sugar range vary. The researchers defined it as between 4.4 to 7.2 millimoles per litre, which is more or less in line with what the UK’s National Health Service and the US Centers for Disease Control and Prevention say.

Although the data differences may seem relatively small between the two experiments, spending a lot of time outside of a healthy range could make a difference to someone’s risk of diabetes complications, such as heart problems, says Hoeks.

As to why sunlight exposure may have this effect, light-sensitive cells in the eye are central to regulating cycles of metabolic activity and are more sensitive to the shorter wavelengths found in natural light, says Hoeks.

Further studies are needed to establish this, he says, but for now, many people with type 2 diabetes could benefit from getting more natural light, even if that means just sitting by a window. “It’s easy, accessible at no cost and available to everyone,” says Hoeks. It is unclear whether people with type 1 diabetes or prediabetes – when someone’s blood sugars are higher than usual, but not high enough for a type 2 diabetes diagnosis – would benefit to the same extent.

Glen Jeffery at University College London says that larger studies are needed to confirm the findings. Nevertheless, he adds that “the importance of daylight is only slowly beginning to be appreciated”.