Just as Ina says.
5-HTP (5-hydroxytryptophan) is the precursor to serotonin, the 'feel good' chemical in our brains. Serotonin seems to get used faster than it's made when we are stressed, or for those of us who are susceptible, during winter. Leading to depression. Taking 5-HTP is very effective for a lot of us in reversing this.
I also take high doses of Vitamin D3, for both the feels-good factor and the immunity boost. Forget the RDA, which is woefully inadequate and out of date, current research suggests 10,000iu is safe and if you look at the amount we make in 10 minutes in the sun, clearly we can handle a lot more. I take 10,000iu a day until it's sunny and warm enough to be outside in short sleeves.
Both these things means that I have the upper hand over depression, especially by the end of the winter, whereas for a lot of years, it had the upper hand over me!