Depressions can be a very serious problem and there is not always a reason behind it. It could be lack of sleep, anxiety, stress or other things that trigger it. When you are depressed your body does not work properly and it could lead to other problems. I know of some people who developed diabetes while being depressed.
I can give you an advise that helped me and it is a kind of way to trick yourself out of depression. It seems very silly and dumb but it actually worked for me.
About 5 times a day, try to find a place to isolate yourself. Then force a smile on your face. Keep it at least 30 seconds. Try not to think about what you are doing and concentrate to keep the smile on. This triggers the production of some hormones in your body that help fight depression.
I know it sounds silly but you really have to force yourself to smile and keep this smile on for as long as possible. There will not be a change immediately but if you manage to this regularly for an extended time, you will feel a difference.
Hope this will help.

Best regards,
Achim