Eating unhealthy foods affects you more than physically — it can have an effect on your mental state as well. Foods high in trans fat, sugar, and even gluten can all negatively impact your mood, according to multiple studies. "Additionally, most people understand what healthy food choices are, so when they make poor food choices, they often feel guilty and experience a decrease in mood," says Gentile.

There's a reason people tend to feel more glum in the winter. Seasonal variations in light patterns that affect the circadian rhythms can cause a decrease in mood and other signs of depression," says Gentile. "Decreased sunlight also causes a decrease in serotonin levels, which often results in a decrease in mood."

If you have a lot to figure out about your life, you might be experiencing “Decision Making Fatigue." "This is the idea that decisions can tire out one’s mind and result in a person feeling overwhelmed as well as experiencing poor concentration and an overall decrease in mood," says Gentile. "Anytime you can make decisions ahead of time, such as choosing what you want to wear the next day or anticipating other decisions you may need to make in the future, can reduce your 'Decision Making Fatigue.'"

Some alone time can be nice, but social interaction is important — maybe even more important than you realize. "As a species, humans are social beings and generally thrive on interactions with others," says Gentile. "When we either intentionally or unintentionally become socially isolated, we can get “stuck” in our own heads. Rather than having a more objective outsider interaction, negative thoughts can spiral and grow. In addition, many social activities involve physical and mental activities that are important for maintaining a healthy and positive mood."

The winter time isn't the only thing that can mess with your body's circadian rhythms — so can staying inside all day, especially if you're in a dark office. "Sitting in a cubicle without windows affects mood," says clinical psychologist Dr. Sanam Hafeez, over email. "Take a break at lunch and get fresh air."

It might be time to log off Instagram and Facebook. A study published in the journal Depression and Anxiety found that heavy use of social media is linked to depression. Whether it's your old classmates political status that pushes your buttons or seeing so many people sharing their most positive life experiences, many people end up unhappy after spending so much time on different sites.

If you're not drinking enough water, you might find yourself more irritable than you would expect. A study from the Journal of Nutrition found that even just mild dehydration can cause moodiness in addition to fatigue.

"The cliche of getting into nature and connecting with it is a cliche for a reason," says Dr. Anjhula Mya Singh Bais over email. "That's why in recent times there has been a spate of how to grow your own garden, even mini herbs in tiny New York spaces," she says. "It's therapeutic."

It might sound counterintuitive, but when you achieve or come into something big or significant, like you win the lottery, get a new job, a bigger living space, new partner, etc., it could leave you feeling unhappy. "It's counterintuitive because most people would assume you would be over the moon and thrilled," says Bais. "Though that may be the case, quite often people get into a funk simultaneously because fear and anxiety surface and grow in relation to maintenance or loss of new acquisition, or even the idea of how to top in the future this recent feather in the cap."

Sleep is important in regulating mood, but you don't want to overdo it. "Getting too much sleep can make you feel grouchy, glum and moody," says psychologist Helen Odessky, Psy.D. over email. "Getting too much sleep does not always have its intended effect, and mood and sleep are related. We feel dull and it affects our mood."

As if physical symptoms aren't bad enough, a hangover can also affect your mental wellbeing. "Overindulging in alcohol can makeyou moody, anxious and irritable the next day," says Odessky. "Alcohol is a depressant, and it affects our cortisol levels — both will affect moodiness."