As we hunker down to endure the middle months of winter, it can often feel like the days are only a few hours long, the nights are so cold you can't go outside, and there's just nothing to do with yourself. But it can sometimes feel like even more than that. You suddenly find yourself in awful moods all the time, and you can't really understand why. At some point in the winter, everyone eventually ends up asking an essential question for human survival: Why does cold weather make you sad?

First of all, take some solace in the fact that you're definitely not the only one who wonders about this. A lot of people struggle with feelings of sadness, or even full-blown depression, during the winter months. Seasonal affective disorder (SAD), a form of depression that happens at the same time every year (usually winter), affects up to two percent of the population, most of them young men and women.

But beyond that, according to HelpGuide.org, up to 20 percent of the population has reported feeling the "winter blues," a more mild form of malaise where you just plain aren't in a good mood.

Seasonal affective disorder is a legitimate form of depression; it's not just about disliking cold weather.

A SAD diagnosis has much less to do with the actual weather, and more to do with the longer nights and shorter days that accompany wintertime, as these things can limit your body's access to sunlight and vitamin D. When winter comes, a lot of things change for your body, including your circadian rhythms, as well as the amount of melatonin (aka your sleep hormone) your body produces in response to the lack of sunlight.

People with SAD usually have to treat their depression in the same way other forms of depression are treated: through therapy, prescribed medication, and/or other forms of homeopathic wellness. Having said that, if you feel like you might really be experiencing this form of depression, it's in your best interest to speak to a professional about it so you can find the right course of treatment for your mind and body.

If you know for sure that your winter blues aren't quite so serious, but you're still looking for a cause and a few ways to cope, there are a few others factors that could explain your meh mood.

One potential cause of your winter sadness might lead to an easier fix: You're not moving your body enough.

When you're dealing with a change in sunlight, and more specifically, your exposure to sunlight, the best way to restore your mood balance and circadian rhythm is to exercise. Sticking to a regular workout routine will help your body produce chemicals like endorphins and serotonin, which will work to counterbalance all of the melatonin your body's pumping out in reaction to these dark days. Try signing up for a winter class special (ClassPass will begin live-streaming workouts in 2018, which might solve this whole problem), and see how a regular sweat sesh changes your daily mood.

Another potential cause of your winter sadness could be less about your body, and much more about your mind.

You might be totally overwhelmed with family obligations, or stressed about your finances after a month of big spending.

The month of December often leads to spending sprees, due to all of the vacations, gift-giving, potlucks, and traveling to visit family and loved ones. Plus, being with family can be just plain tough in general. No one would judge you if spending 10 days with extended family made you a little more moody than usual.

For these issues, January is the ultimate month to take your life back into your own hands. Because it's so damn cold almost everywhere (I'm in Los Angeles, and even I'm currently wearing two sweaters), January is a great month to reassess your spending, prioritize what you want to spend money on over the next few months, and maybe even do a social media or phone detox where you can have a few days of silence after non-stop small talk since Thanksgiving.

There are countless reasons for why it can get a little bit harder to see the light (literally) during this time of year. But the good news is that all of these struggles are treatable, in one form or another.