Improve your mood by going straight to the source. Serotonin is the neurotransmitter - a chemical messenger in your brain - that most scientists attribute to feelings of happiness. Go ahead and reach for the chocolate. Upping your body's levels can be as simple as adopting a few new habits - and snacks!

When you're having one of those days, you likely turn to chocolate. This dark indulgence not only tastes good, but it can actually boost your mood by increasing your levels of serotonin. Aim for about 1 ounce (or 200 calories) per day, advises Jaclyn London, M.S., R.D., C.D.N., Nutrition Director at the Good Housekeeping Institute.

Take a deep breath. Mindful meditation - meditating while acknowledging your thoughts and accepting them without judgment - may improve physical and emotional health by reducing anxiety, chronic pain and high blood pressure. It can also increase the concentrations of grey matter in the part of the brain responsible for regulating emotions, according to a small study in Psychiatry Research. These changes may occur due to a positive impact on serotonin production, scientists say.

Yes, they're important but so is exercise. Regularly getting up and moving - even if it's just walking more frequently - can help with insulin sensitivity, London says. Your ultimate goal: 10,000 steps per day, which sounds like a lot but adds up quickly if you make a point of moving throughout the day.

There's a reason why a sunny day can lift our spirits. The surge of feel-good hormones released during exposure to ultraviolet radiation can actually trigger an addictive-like behavior, according to a 2014 study done on mice by Massachusetts General Hospital and Harvard Medical School. The researchers believe these naturally-occurring chemicals activate the same pathway as painkillers like morphine.

The next time you need a jolt of joy, make sure your dinner includes some omega-3 fatty acids. "Omega-3 deficiency has been linked to alterations in serotonin and dopamine levels," explains Dr. Mercola. "Studies show there's a strong association between low omega-3 levels and depression." You can find these healthy fats in foods like fish, walnuts, grass-fed beef and dairy.

Yogurt and fermented foods (like sauerkraut and kimchi) have already gotten a lot of attention for their gut-boosting live cultures, but probiotics are popping up in other places too. You can now find the bacteria in chips, cookies, and granolas, but like bug-based foods, make sure you're looking at the other ingredients before adding to your cart.

If you're not a fan of fermented foods, Dr. Mercola says a high-quality probiotic supplement can also help your microbiome and optimize the production of pleasure-related chemicals. "Some researchers have even proposed probiotics are the new Prozac, considering their beneficial effect on psychiatric health," he states.

When you're craving chocolate, indulge in some one-on-one time with your partner instead. "Falling in love, being in love and making love all boost our serotonin," states Dr. Jenn Mann, licensed psychotherapist. "They also trigger the release of oxytocin, these wonderful brain chemicals that elevate our mood." You can experience this euphoria even without going all the way. "Kissing, hugging, cuddling, spooning together, making out and holding hands will achieve the same results," she adds.