Why you should sleep with lights off during the night

By Roberta Attanasio

In cities and towns all around the world, the nighttime glow of artificial light obscures views of the cosmos and contributes to an environmental problem called “light pollution.” Light pollution can also be present at smaller scale, for example in neighborhoods, or in homes. Often, this problem is caused by superfluous light. Solutions such as covering streetlights to direct beams downward or turning off unnecessary lights can be very effective. But why do we need to think about solutions?

“For billions of years, biology evolved in a world where light and dark was controlled by the length of the day. When the sun went down, celestial sources like the moon, stars, planets and Milky Way lit the sky. Life learned to operate under their glow. Only in the last 100 years or so—with the spread of artificial light—has that cycle largely gone away.”

The 24-hour light and dark cycles are at the basis of our circadian rhythms—the internal clock that guides day and night activities and controls physical, mental, and behavioral changes in nearly all living organisms. Because of light pollution, we are now in a circadian fog where our physiology is confused—the circadian system is not getting a clear signal of day versus night.

Notably, light pollution affects the production of the hormone melatonin, which is released when it is dark and is inhibited when there is light present. Increased exposure to light at night lowers melatonin production, resulting in sleep deprivation, stress, fatigue, headaches, and other health problems.

However, there is more to it. A new study (published March 14, 2022) shows that even moderate light exposure during sleep harms heart health and increases insulin resistance. Senior study author Phyllis Zee said: “The results from this study demonstrate that just a single night of exposure to moderate room lighting during sleep can impair glucose and cardiovascular regulation, which are risk factors for heart disease, diabetes and metabolic syndrome. It’s important for people to avoid or minimize the amount of light exposure during sleep.”

The study tested the effect of sleeping with 100 lux (moderate light) compared to 3 lux (dim light) over a single night. The investigators discovered that moderate light exposure caused the body to go into a higher alert state called sympathetic activation. In this state, the heart rate increases. The force with which the heart contracts and the rate of how fast the blood is conducted to blood vessels also increase.

Interestingly, earlier studies showed that exposure to artificial light at night while sleeping may be a risk factor for weight gain and development of overweight or obesity. Results from the new study provide an explanation for the previous findings. The researchers found that insulin resistance measured the day after people slept in presence of moderate light was higher than that of people who slept in dimly lit rooms.

Insulin resistance is a complex condition in which muscle, fat and liver cells do not respond as they should to insulin, a hormone made by the pancreas and essential for regulating blood sugar levels. In other words, these cells become resistant to the action of insulin, eventually causing elevated blood glucose levels and, over time, prediabetes and Type 2 diabetes.

Exposure to indoor and outdoor nighttime light is widespread. The researchers recommend to take preventive measure as for example closing the blinds, turning TV and lights off, or wearing eye masks while sleeping.  

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