Light at Night Can Harm Your Memory

Being exposed to bright lights at night could harm your health and your memory - even if you think you are sleeping normally...

Tara LeGates and her colleagues from John Hopkins University, Baltimore, exposed a group of mice to an alternating pattern of 3.5 hours of bright light followed by 3.5 hours of darkness over a 2 week period.  This produced no change in their sleeping pattern, the overall length of time they slept for, or the levels of hormones that usually change at different times of the day.

But it did make the mice depressed, and reduced their abilities to form new memories.  Depressed mice demonstrate behavioural signs, such as lower preference for sweet things.  This can be an indicator of 'anhedonia'- the inability to feel pleasure, which is a symptom of depression in humans.  Depressed mice also "give up" on certain tasks more readily, like poorly motivated people.

Humans with depression also often complain of memory problems. These mice were the same, establishing new memories less well than control animals.

This experiment meant that when the mice - which are normally nocturnal - were waking up to become active in what should be night-time, and dark, they were actually being exposed to light. And this - being exposed to light when the brain is not expecting it - the researchers think, is what made the mice depressed.

LeGates and colleagues, writing in the journal Nature, think that it's down to specialised light-detecting cells in the eye called intrinsically photosensitive retinal ganglion cells.  These cells don't help animals to see but instead they detect when it is daytime or night-time, and they're used to set the brain's internal clock circuitry that enables animals to know when it's time to wake up or to go to sleep.

But, the researchers found, these cells also send signals to the parts of the brain concerned with mood and memory.  And being exposed to light at the "wrong time" - so in the case of the mice, when they were awake and expecting it to be dark; for a human this would be the equivalent of working late at night in front of a bright computer screen - abnormally triggers these mood-related brain areas, causing depression, probably by provoking increasing the levels of the stress hormone cortisol.

Humans have the same light-sensitive cells as mice, so there is no reason to think that this couldn't apply to us.  It is something important to bear in mind particularly for those who undertake shift- work, or regularly who work on computers late at night.  So ignore your bed time and leave the lights on at your peril!