A study that was published in the March 2018 Journal of Epidemiology directly linked dim light at night with an increased risk of depression.

Let me repeat - dim light at night was found to increase the risk of depression by 89%.

The researchers followed a group of older men in Japan for two years. They found that those who were exposed to more than 5 lux of light at night were much more likely to be diagnosed with depression.  

So what is 5 lux of light? It is about the amount from a nightlight – less than leaving the TV on but more than moonlight.

This is just one study, and while it looks like the researchers accounted for all the confounding factors, let's see what else is available to back up this idea that dim light at night is a risk factor for depression:

First, a study that shows that there is a measurable impact on the brain from dim light at night.
A 2016 study investigated the impact of dim light at night over a three night period. The study participants slept in a sleep lab for two nights under no light and then on the third night they were randomized to either sleep with 5 or 10 lux of dim light. The next morning, study participants were put through a series of tests while their brains were being imaged in an fMRI machine. Those who had been exposed to 10 lux of light had less activity in their frontal cortex, even though reaction time wasn't statistically significant.  So one night of dim light exposure at 10 lux is enough to show a difference in a brain scan.  

Next up, a study showing that dim light at night impacts sleep structure. The study in 20 - 30 year olds found that sleeping exposed to dim light at night "during sleep increases the frequency of arousals, amount of shallow sleep and amount of REM sleep."

But why would dim light at night matter if you are asleep? Light can actually go through your eyelids and affect melatonin levels. There have been several studies using sleep masks with blue LED's built in that deliver a pulse of light every minute or two. This has been shown to effectively decrease melatonin and shift circadian rhythm - which would be good for shifting time zones in jet lag.

Animal studies are also good for giving us an idea of how dim light at night could cause these changes. A study using hamsters who are diurnal (active in the daytime) found that dim light at night increased TNF, an inflammatory cytokine, in their brains. This caused reversible depressive symptoms.  Luckily, there is another study showing that depressed hamsters due to dim light at night respond well to the antidepressant Celexa, which works on circadian rhythms. (Yes, I'm being a bit sarcastic here – perhaps just eliminating light at night would be a better idea than giving the depressed (hamsters) an antidepressant.)

What can you do to prevent dim light at night?

First, figure out where the artificial light at night in your bedroom is coming from.

If it is light from nearby streetlights or just light pollution from an urban area, get some blackout curtains. You may need to stack blackout shades along with blackout curtains if you live where a streetlight shines in your window.  

On a budget or already have curtains hanging that you love? Hit your local thrift stores to see if they have inexpensive blackout curtains available. They may not be your style, but you can double hang them behind your current curtains for an inexpensive fix.

If the light is coming from within your home, find a way to shut it down. Put electrical tape over all the LED indicator lights on your chargers. Make sure your electronics are shut off at night.

If you live in an apartment or flat and have light seeping under your door, you could use a draft dodger to block it out.  Or just roll up a towel and put it in front of the door.

Draft dodger on Amazon is a nice looking option.

Can't put up curtains? Get a sleeping mask.  
If you are in a situation where you can't put up curtains or blackout shades, a sleeping mask will make a big difference.  There are all kinds of options these days including ones made of pure silk – and at a reasonable price. A definite "must have" for traveling as well.

Pure silk sleep masks on Amazon.

Hopefully this has given you some ideas of what will work for making your sleeping environment completely dark.