Sleeping with a light on can play havoc with insulin levels and consequently impair the response to glucose, a two-night sleep-lab study of 20 people indicates.

"The most important finding" is that compared to one night in a dim light environment, "one night of exposure to a moderate level of room light while sleeping with eyes closed increased heart rate and sympathetic [nervous system] activity during the entire sleep period," said senior author Phyllis C. Zee, MD, PhD.

And on the morning following the moderate room light condition, a higher amount of insulin secretion was required to normalize glucose levels following ingestion of a bolus of glucose in an oral glucose tolerance test, consistent with higher insulin resistance, Zee, Director of the Center for Circadian and Sleep Medicine at Northwestern University, told Medscape Medical News in an email.

The study by Ivy C. Mason, PhD, also of Northwestern University, and colleagues was published March 14 in the Proceedings of the National Academy of Sciences.

Melatonin levels were similar under the two light conditions, Zee added, which "suggests that the effect of light during sleep on these cardiometabolic measures were more likely due to activation of the sympathetic [nervous] system and less likely due to changes in sleep or suppression of melatonin by light."

"Attention to avoiding exposure to light at night during sleep may be beneficial for cardiometabolic health," the researchers conclude.

That means "turn lights off before sleeping," Zee elaborated. If a light is needed for safety reasons, keep it as dim as possible, she advises, and avoid exposure to blue or green light, but instead try red-amber colors.

Several studies have investigated the effect of light on sleep and metabolic outcomes, the researchers explain.

In one study, light in the bedroom was associated with obesity in women, and in another study, it was associated with risk of type 2 diabetes in an elderly population.

Research has suggested that nighttime light exposure may alter glucose metabolism by increasing insulin resistance; lowering melatonin levels, which alters insulin secretion; and having an arousing effect on the sympathetic autonomic nervous system (increasing the stress hormone cortisol or heart rate, and decreasing heart rate variability).

However, the effect of a single night of moderate room light exposure across the entire nighttime sleep period has not been fully investigated.

The researchers enrolled and randomized 20 healthy young adults who were 18 to 40 years old and regularly went to sleep between 9 PM and 1 AM and slept 6.5 to 8.5 hours, to sleep two nights in the sleep laboratory under two conditions.

Lights On During Sleep Can Play Havoc With Metabolism

Ten participants (8 women, 2 men) slept in a dim light condition on Night 1 and in a moderate light condition on Night 2. The other 10 participants (6 women, 4 men) slept 2 nights in the dim light condition.

The moderate light condition consisted of four 60-watt incandescent overhead ceiling light bulbs (a total of 100 lux), which "is bright enough to see, but not to read comfortably," Zee explained. "It's like hallway light in an apartment. But the people were sleeping, so about 90% of the light would be blocked by the eyelids."

The dim light condition was less than 3 lux, which is dimmer than a night light.

When participants were awake, the room lighting was 240 lux.

Participants in each group were a mean age of 27 years and had a mean body mass index of 23 and 24 kg/m2.

The week before the study, participants went to bed at 11 PM and slept for 7 hours (based on actigraphy measures). During the laboratory stay, the participants were allowed to sleep 8 hours, during which polysomnography was performed.

They received standard meals at 2.5, 5, and 11 hours after waking and had 30 minutes to eat them. Snacking or caffeine were not permitted.

Participants were instructed to remain seated or standing in their room, but not exercise, when they were not sleeping. Blood samples to determine melatonin levels were collected hourly during wake and sleep via an intravenous line.

Participants slept for a similar time, around 7 hours, in both conditions.

Although melatonin levels were similar in both conditions, this was a relatively small sample, the researchers caution.

In the room light condition, participants spent proportionately more time in stage N2 sleep and less in slow-wave and rapid eye movement sleep. There was no increase in sleep fragmentation or arousals.

The research was partly supported by the Center for Circadian and Sleep Medicine at Northwestern University, the National Center for Advancing Translational Sciences, the National Institutes of Health, and the American Heart Association. The researchers have reported no relevant financial relationships.

PNAS. Published online March 14, 2022.

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