Let There Be Light!

When I was an undergraduate student, I was an expert at pulling “all-nighter” study sessions prior to exams and project deadlines.  Once everything was said and done the next day, many of my classmates moved on to celebratory happy hours, while the only thing that could make me happy at the time was heading to bed!  Even after some rest, however, I was slow, lethargic, and felt misplaced.

It is not unusual to experience similar changes in mood when you travel far away or when daylight savings time kicks in.  Indeed, light is an important player in synchronizing our circadian rhythms and sleep-wake cycles.  Disrupting either sleep or circadian rhythms by altering light-dark cycles has been shown to affect mood and cognition.  Until now, it was not clear whether light itself or the changes induced by light underlie these effects.

A research group from Johns Hopkins University recently showed that light directly impairs mood and cognitive functions.  The results, published in last month’s edition of Nature Neuroscience, show that mice subjected to an aberrant light cycle of 3.5-hour light and 3.5-hour dark (T7) (as opposed to the normal 12-hour light and 12-hour dark T24 control cycle) had adverse effects on behavioral performance as well as learning.

This aberrant light cycle, which does not affect total sleep time or the body’s circadian clock, was shown to induce depression-like behaviors and measures in mice.  Elevated levels of corticosterone, a steroid hormone that has been linked to depression, were seen in mice living in the T7 light cycle.  In an anhedonia test, when mice were provided with sucrose water, T24 controls preferred the sugar-water to regular water, but T7 showed no preference, indicating an increase in depression-like behavior.  Results from the forced swim test showed heightened behavioral despair in mice living in the T7 light cycle .

The aberrant light cycle also induced learning deficits in mice.  The T7 mice did poorly in the Morris Water Maze, a task used to study spatial learning and memory, and in the novel object recognition test, a task used to assess hippocampal-dependent memory.  There were also deficits in long-term potentiation (LTP), a measure of the strength of the connection between neurons which has a mechanism correlated with learning and memory.  Taken together, these findings suggest that aberrant light cycles induce symptoms of depression disorders, which inhibit their ability to learn.

The researchers then attempted to treat the T7 mice with the antidepressant fluoxetine, and amazingly, the treatment reversed both the depression-like behaviors and the learning and memory deficits previously observed!  The research group determined that a population of neurons called intrinsically photosensitive retinal ganglion cells, projecting from the eyes to areas of the brain that are responsible for regulating circadian and sleep cycles, have an essential role in the subconscious detection of light and ultimately may be responsible for the depression and learning deficits observed in individuals with disruptive light-cycles.

This study provides evidence that light directly impacts mood and cognition and introduces a potential mechanism by which major depression disorder may exert its devastating symptoms.  It also shows that current antidepressant medications are effective in alleviating these debilitating symptoms.

If final exams, projects, or holiday stress become overwhelming, just make sure you’re exposed to proper light-cycles by maintaining healthy sleeping amounts and patterns.  Looking back, all-nighters did not always set me up for my best performances and cognitive success!