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Light at night — even low levels — is linked to increased risk of depression

Light at night — even low levels — is linked to increased risk of depression
The researchers say it’s not clear how light at night may be linked to depression, but they note that it’s probably linked to light’s impact on the release of melatonin.

Sleeping in a room with even a low level of light is associated with an increased risk of developing depression, according to a study from Japan.

The study also found that the link between light exposure and depression occurred even when the light did not cause people to awaken or experience other sleep disturbances during the night.

The research suggests, say its authors, that sleeping in total darkness may be a “a novel and viable option to prevent depression.”

Study details

For the study, researchers at the Nara Medical University School of Medicine in Nara, Japan, recruited 863 older adults (aged 60-plus). None of the participants had symptoms of depression at the start of the study, and most were retired.

Light meters were placed at the heads of the participants’ beds for two consecutive nights to capture the light in their rooms as they slept. The participants also wore a sleep-monitoring device that recorded details about their sleep/wake patterns, including when they fell asleep. In addition, they filled out sleep diaries, and answered questions about their health and lifestyle habits.

The participants were followed for two years for signs of depression. During that period, 73 of them were diagnosed with clinical depression. The researchers then looked to see if there was any connection between those who developed depression and the amount of light in their rooms at night. They found that people who were exposed to five lux or more of light at night had a “significantly higher depression risk” than those who slept in darker rooms. (Five lux is roughly the equivalent of the light that falls on a three-foot-square surface that is about three feet away from five candles.)

The finding held even after the researchers adjusted for a variety of other factors, including age, gender, income, body mass index (BMI) and existing chronic illnesses, such as high blood pressure and diabetes. Nor did the finding change when sleep-related factors were considered, such as when the participants went to bed, how long they stayed in bed and whether their sleep was disturbed.

The researchers say it’s not clear how light at night may be linked to depression, but they note that it’s probably linked to light’s impact on the release of melatonin, a hormone known to play a major role in the circadian (daily) sleep-wake cycle.

“Previous studies have suggested the possibility that LAN [light-at-night] induces sleep disturbances, impaired melatonin secretion and misalignments between sleep/wake behavior ... and depression is frequently accompanied by these conditions,” they write in their paper. 

Younger people at risk, too

The study has several limitations. Most notably, the participants weren’t randomly selected, so they may not be representative of the general population. In addition, light intensity was not measured at eye level and only over two days. Furthermore, some of the light measurements were taken on the weekends, which may have represented atypical exposures. 

Still, the findings are intriguing, as they support other research that has found a higher risk of depression among night-shift workers that is significantly correlated with exposure to light at night and disruptions in the body’s daily biological rhythms. A growing number of studies have also linked bedtime exposure to blue light (from smartphones, tablets and other electronic devices) with decreased quality of sleep and changes in mood.

The authors of the current study also point out that the association they found between light-at-night exposure and depressive symptoms is unlikely to be limited to older populations. In fact, quite the opposite may be true.

“Age-related cloudiness of the crystalline lens causes decreased light reception to the retina, even before cataract diagnosis,” they explain, “and the capacity for light reception of a 70-year-old is one-fifth of that of a teenager.”

Furthermore, the area in the brain that functions as the master pacemaker of the body’s internal circadian rhythms — the suprachiasmatic nucleus — becomes weaker (and thus less susceptible to the influences of light) with age. 

“Together, these findings imply that younger individuals might be more sensitive to [light at night] than are the elderly,” the researchers conclude.

FMI: The study was published in the March issue of the American Journal of Epidemiology, but is, unfortunately, behind a paywall.

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