Everyone is sometimes down
Does that mean we're depressed? No, circumstances and situations can put a heavy burden on us that we don't like it anymore for a while. This is a form of depression that everyone experiences. If you are regularly in the pit, or if that is a pattern that often recurs, it is possible that your hormonal system is not in balance or that your nervous system is out of balance. This can be hereditary. But it doesn't mean that you are really depressed.
Natural daylight can also play a role in how we feel. This form of depression is more common in the dark period of the year. Apparently this has to do with the epiphysis. Certain forms of light therapy are applied to these problems. Also nutrition is important, also movement, our relations, our work et cetera. Depression often has a multicausal basis. In that case you have to look at what causes this depression to do something causal about it.
Real depression is a serious problem, a serious illness that requires specialized help.
It is the inability to show an adapted emotional life. If there is nothing left to bring any joy, this is a signal to seek medical or psychological help.