By all means talk to your doctor if you feel you are seriously depressed - but know there is help available for those days when you feel blah and just can't shake it.
First off, If of the female gender, I ALWAYS will suggest taking a stab at balancing the hormones. I use specific female herbs like Dong Quai, Wild Yam, Chaste Tree, Angelica and a few others to have mother nature assist! So many factors will cause hormones to go up and down. If still getting periods, there can be such a drop before your cycle begins that you will feel it and it can feel like you are sinking - either that or your moods will be a roller coaster. With Peri Menopause and Menopause - it isn't as drastic but can be more of a daily cloud that comes and goes. If you are of the male species, and over 40 - also can't hurt to have your testosterone levels checked as they tend to go down or fluctuate causing changes as well.
Secondly - What are you eating? Have you gotten into a habit of just grabbing food and not really enjoying it? More takeout than home-cooked meals? Not taking supplements to offset your sometimes poor food choices? This can be another way to soothe the moods - Yes you really DO feel better when you eat better. You will actually be getting real nutrition into your blood stream, cells and organs. Fruits, veggies, salads - easy on the body, full of nutrition and energy inducing benefits. Try and cut back on the quick fixes.
Third - Get the pipes moving! Yes - improve your bowel habits if you aren't going once or twice per day. Even if you are, one weekend a month of cleansing the bowel can be HUGE to release stored up toxins and improving your mood. If you aren't going daily then gentle cleansing will feel like a weight has been lifted. You may even start skipping without realizing it- lol.
Last but not least - Movement and Nature. Move the bod and get some fresh (outdoor) air!! I don't care how cold it is. I know I don't like to be cold. But there's something so invigorating about being bundled up properly and going for a walk in the winter. (I do tend to pass if windy) In Western NY we don't care what we look like if it's a matter of keeping warm! lol (I'm glad it's pretty well understood here!) Warmth is more important. Also I keep tissue stuffed in my pockets as my nose runs almost immediately when I go out...but other than that...it's such a treat to be walking/moving to the point of where I feel warmth. I have some good walking boots that keep me warm and have good traction, so really there's no excuse. But what it does for the soul and mind - being out in nature, revving up the circulation and just 'moving' - really takes the edge off any prior dreary mood.
You don't have to settle for feeling down or depressed any time of year, but especially not during our longer winter months. Lots you can do to feel good, so have these ideas in place the next time you feel a black cloud has found you - the fog will lift sooner when you know what to do about it!