Serotonin helps to keep our mood boosted and give us energy and when we lack serotonin it can cause use to feel depressed, pessimistic, and have irregular appetite and sleep.
When our serotonin levels increase then we usually feel much better and are happier, in a better mood, get better sleep etc.
When people lack serotonin they also sometimes become irritable, anxious, and perceive the world as unfriendly.
Several things including lack of sunlight and other things such as genetic factors, faulty metabolism, and digestive issues can impair absorption and breakdown of our food which reduces are ability to build up serotonin levels.
A doctor can sometimes prescribe medication to help get your serotonin levels back to where they should be so you should see a doctor if you think your serotonin levels are too low or dropped.
You'll much better when you have your serotonin levels increased.