Numerous studies have looked at exposure to artificial light the night before and during sleep revealing a shifting effect on the biological clock, detrimental in particular to metabolic health, with in some cases weight gain. Exposure to artificial light at night during sleep has become common, whether from screens or from sources outside the home, especially in large urban areas. It is also estimated that around 40% of people sleep near a light source.
This new study shows that sleeping in a room, even dimly lit, increases the risk of heart disease and diabetes: with this exposure, even moderate, the heart rate increases and the body cannot rest.
The heart needs darkness too to rest
The Chicago researchers compare here, on a single night, in a sleep laboratory, the effects of sleep exposed to 100 lux (moderate light) vs 3 lux (low light) during a single night. They find in participants that:
- moderate exposure to light puts the body into a heightened state of alert;
- in this state, the heart rate increases, via the activation of the sympathetic nervous system, as well as the force of cardiac contraction and the speed at which blood is propelled to your blood vessels;
- insulin resistance is observed from the morning following a night of sleep exposed to moderate light; in compensation, the pancreas produces more insulin with a longer term risk of increased blood sugar;
- when questioned, the participants declare that they are not aware of these biological changes in their bodies during the night;
“But the brain feels it”,
comments lead author Dr. Daniela Grimaldi, a neurologist and researcher at the Center for Circadian and Sleep Medicine at the Feinberg School of Medicine: “The brain then behaves as during a sleep is light and fragmented. The physiology of sleep does not calm down as it should”.
A single night of light exposure is enough: a single night of exposure to moderate ambient light during sleep can impair glycemic and cardiovascular regulation, and exacerbate these risk factors for heart disease, diabetes and metabolic syndrome, says Dr. Phyllis Zee, co-lead author of the study. There are sympathetic and parasympathetic nervous systems to regulate our physiology during the day and night. The sympathetic takes charge during the day and the parasympathetic is supposed to do so at night, when it transmits restoration to the whole body.
Light and its relationship with health are a double-edged sword: in addition to sleep, nutrition and exercise, exposure to light is an important health factor, but during the night, light turns into a risk factor by altering heart and endocrine health. The researchers therefore simply recall:
- Don’t turn on the lights. If you need to have a light on (which older people may want for safety), choose a dim light set at ground level;
- opt for a less brain-stimulating amber or red/orange light. Avoid white or blue light;
- the use of masks to obscure the eyes can be effective if it is not possible to control all light sources;
The justice of the peace is not being able to distinguish the objects around you: “if you can make out things just fine, it’s probably too bright.” Close the blinds, draw the curtains and turn off all the lights before going to bed”.