Light exposure during sleep may increase diabetes risk

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Light exposure during sleep may increase the risk for heart disease, diabetes, and metabolic syndrome. Gregor Prelog/EyeEm/Getty Images
  • Researchers recently investigated the health effects of light exposure during sleep.
  • They found that light exposure during even one night of sleep increases heart rate while sleeping and impairs glucose metabolism the following morning.
  • They say that sleeping without exposure to light is likely beneficial for cardiometabolic health.

Exposure to artificial light at night is widespread globally and is linked to negative effects on health and well-being.

Researchers from Northwestern University in Chicago recently investigated the biological effects of different light exposure levels while sleeping.

They found that exposure to moderate ambient light during one night of sleep can impair glucose and cardiovascular regulation and increase risk factors for heart disease, diabetes, and metabolic syndrome.

The study appears in PNAS.

Another study, published in 2019, found that artificial light at night (ALAN), such as from a nightlight or television, is linked to obesity in women. These findings suggest that light exposure during sleep may negatively affect metabolic regulation.

Other research indicates that blue-enriched light exposure in the morning and evening alters glucose metabolism and increases insulin resistance compared with dim light exposure.

Another study found that higher levels of ALAN are linked to higher rates of type 2 diabetes among older adults.

The mechanisms underlying ALAN’s impact on metabolic functioning are poorly understood.

The scientists recruited 20 young adults for a 3-day and 2-night laboratory stay. A week before the study, they measured participants’ sleep habits using actigraphy and sleep diaries.

The participants were then randomized to partake in one of two sleeping conditions:

  • The room light condition: Participants slept with a dim light of less than 3 lux (lx) on the first night and an overhead room light at 100 lx on the second night.
  • The dim light condition: Sleeping with a dim light of less than 3 lx on both nights

On Days 1 and 2 of the experiment, the participants provided blood samples before and after food intake to assess melatonin and blood glucose levels. They also underwent overnight polysomnography (PSG) to assess their sleep quality.

The participants also filled in surveys every 2 hours that they were awake to evaluate their subjective sleepiness, hunger, and mood changes.

Meanwhile, nurses collected blood pressure readings every hour, alongside heart rate readings every 4 hours.

The researchers found that exposure to a single night of 100 lx room lighting increased heart rate during sleep and insulin resistance the following morning.

Compared with the dim light condition, participants exposed to 100 lx lighting during sleep had more N2 sleep — one of the deeper stages of sleep — and less slow-wave sleep — which is important for memory consolidation. They also had less REM sleep — the dreaming phase of sleep.

The researchers noted no changes among PSG-derived measures of cortical arousal, sleep fragmentation, or wake-sleep stage stability and no difference in melatonin levels between the two groups. They say this might be because only 5-9% of light is transmitted through closed eyelids.

When asked what may explain the link between ALAN and the observed effects on heart rate and glucose metabolism, Phyllis C Zee, MD., Ph.D., the correspondent author of the study, told Medical News Today:

“There are potentially three possibilities. [The first is that light] can cause awakening or arousal. We found small changes [in these measures, but they did not] correlate with an increase in insulin resistance. So this is unlikely to be a primary mechanism.”

“Secondly, light can affect the circadian clock and affect melatonin secretion. However, we found no significant difference in melatonin levels,” Dr. Zee added.

“[We thus hypothesize] that light activates brain regions that regulate the autonomic nervous system because there was a relationship between the changes in heart variability and insulin resistance,” she explained.

Dale Sandler, Ph.D., is a senior investigator and chief of the Epidemiology Branch of the National Institute of Environmental Health Services (NIEHS), who was not involved in the study. Dr. Sandler agreed with Dr. Zee but added that “subtle shifts in sleep stages could [also] play a role.”

The researchers concluded that avoiding ALAN may benefit cardiometabolic health.

When asked what limitations there may be to this research, Yong-Moon (“Mark”) Park MD, MS, Ph.D., assistant professor at the University of Arkansas, who was not involved in the study, toldMNT:

“The findings came from a study of 20 healthy young adults. So the results might not apply to people with different age groups and people with comorbidities. [Also]the effect of ALAN on cardiometabolic health may differ by sex and race/ethnicity, which was not addressed in this study.”

“As the authors mentioned, the effects of altering light intensities, duration, and wavelengths on cardiometabolic health were not assessed in this study. The [link between blue light and cardiometabolic dysfunction] may be important to look at in the future as the use of smartphones, tablets, and laptops have increased in the bedroom at night,” he added.

“The experiment only involved one night of sleep in room light for the exposed group,” added Dr. Sandler, “Obesity and diabetes do not develop overnight. What may be most important is long-term exposures, and it is possible that some effects that were not observed after one night could develop over constant longer-term exposure.”

“There are many things that contribute to risk for obesity, diabetes, and cardiometabolic dysfunction. Many of these factors are difficult for individuals to control. But turning off the lights at bedtime may be a simple thing that people can do to lessen the chances of developing these conditions.”

– Dr. Sandler

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