well usually when I start feeling bored and depressed from not doing anything, I just think of something I'd enjoy and decide to it. Study something, watch a show, start a run of a game I like, etc.
Eating something helps too but only cause I'm a skeleton and only notice Im hungry when I start feeling listless and depressed out of nowhere.
Iunno what you do if you're too listless to just make yourself do something, and its cause of an actual condition rather than just needing to eat or something.