JOHNS HOPKINS (US) — Routine exposure to bright light at night may increase our risk of depression and learning issues, experiments with mice show.
The study strongly suggests that abnormal light exposure itself can directly change mood and cognitive ability.
The late-night light, the researchers say, acts independently of sleep deprivation and disruption in the body’s circadian rhythms, which already are known to have similar effects.
“Basically, what we found,” Johns Hopkins biology professor Samer Hattar says, “is that chronic exposure to bright light—even the kind of light you experience in your own living room at home or in the workplace at night if you are a shift worker—elevates levels of a certain stress hormone in the body, which results in depression and lowers cognitive function.”
Hattar and his team demonstrated how special cells in the eye (called intrinsically photosensitive retinal ganglion cells, or ipRGCs) are activated by bright light, affecting the brain’s center for mood, memory, and learning.
Their results were published in the journal Nature.
The study involved mice, but the findings are very likely applicable to humans, Hattar says.
“Mice and humans are actually very much alike in many ways, and one is that they have these ipRGCs in their eyes, which affect them the same way,” he says.
“In addition, in this study, we make reference to previous studies on humans, which show that light does, indeed, impact the human brain’s limbic system. And the same pathways are in place in mice.”
Hattar’s team, led by graduate students Tara LeGates and Cara Altimus, exposed laboratory rodents to a cycle consisting of 3.5 hours of light and then 3.5 hours of darkness.
Previous studies using this schedule showed that it does not disrupt the mice’s sleep. It did, nevertheless, cause the animals to develop depression-like behaviors, the researchers found.
“Of course,” Hattar says, “you can’t ask mice how they feel, but we did see an increase in depression-like behaviors, including a lack of interest in sugar or pleasure-seeking, and the study mice moved around far less during some of the tests we did.
“They also clearly did not learn as quickly or remember tasks as well,” he adds. “They were not as interested in novel objects as were mice on a regular light-darkness cycle schedule.”
The animals also had increased levels of cortisol, a stress hormone that has been linked in numerous previous studies with learning issues.
Treatment with Prozac, a commonly prescribed anti-depressant, mitigated the symptoms, restoring the mice to their previous healthy moods and levels of learning, and bolstering the evidence that their learning issues were caused by depression.
The results suggest that humans should be wary of the prolonged regular exposure to bright light at night that is routine in many lives, because it may be having a negative effect on our mood and ability to learn, Hattar says.
“I’m not saying we have to sit in complete darkness at night,” he says, “but I do recommend that we should switch on fewer lamps and stick to less-intense light bulbs. Basically, only use what you need to see. That won’t likely be enough to activate those ipRGCs that affect mood.”
Funding from the David and Lucile Packard Foundation supported the work.
Source: Johns Hopkins University