Everyone knows that winter is the time when you shut yourself indoors, surround yourself in a heap of blankets, sip a nice hot beverage, and put on a cheerful movie/read a cheerful book.

However, for some people, they shut themselves indoors for a completely different reason.

It's because they have SAD, or Seasonal Affective Disorder. What is SAD? Basically put it's, well, feeling sad, but at certain times of the year. For most people, this time is the winter due to the decreased amount of sunlight, lower temperatures, and lack of plants growing outside (which are actually known to benefit mood).

However, some people experience SAD at times of the year that serve as particularly traumatic for them, such as the time of year a loved one passed away.

In regards to winter-related SAD, the effects can be varying. They can range from slight irritability to complete isolation and depression. Sunlight directly affects mood, and this is perhaps the biggest reason people get the blues in the winter. Some direct ways to combat the winter SADness are light therapy and a sunny vacation closer to the equator. However, if you're like most people, these things don't come so easily.

If you notice these SAD characteristics in a friend, such as the friend not making plans to meet up as often as they used to, or preferring to be alone when they normally wouldn't, understand that they are not being intentionally rude to you. If you can, go the extra mile to try to get them to do things with you.

It can just be asking them to coffee or lunch! Social interaction is proven to increase happiness and self confidence. If you can't physically see them, at the very least shoot them a text about how they are doing. You never know how much this can mean to a person, especially if they are going through a funk.

Fun factoid: the modern holiday season was actually invented to combat this winter depression! "The holiday season was invented?!" you say. Why, yes. Yes, it was!

Picture this: festivities in mid to late December, focusing on feasts of food, gift-giving, decorations, charity towards the poor, closing of businesses, and celebration of a deity. Sounds like the Christmas season, right?

Sure, it does, but it actually predates Christmas. The original festival that had these characteristics was known as Saturnalia. This Roman festival paid homage to the god Saturn, and conveniently came at the time of the year when people were growing sour due to the lack of food, because, obviously, not much would grow in the winter. The festival gave the Romans something to get their mind off the dreary winter and into party mode.

In comes Christianity. It may seem crazy now, but early Christians didn't really hype up Christmas. Easter was their big holiday. (After all, everyone is born, but shouldn't we make a big deal about someone rising from the dead?!) However, when the Roman Empire became predominantly Christian, Romans couldn't very well worship the old gods anymore, now could they?

So, they needed a replacement. Since nobody knew for sure what time of the year Jesus was born, they decided to make it in December. Most of the Saturnalia and even January-related Roman holidays were combined to form the celebration of Christmas. (Though scholars of the Bible put Jesus's actual birth at sometime in the summer, according to the census at that time, which was the reason Mary and Joseph went to Bethlehem)

So, the most wonderful time of the year actually comes from people trying to cheer up about the most awful time of the year.

So, long story short, if you're feeling SAD, go to a holiday party! They were invented for this very reason.