I can sympathise with all of you who have “down days” - less “black dog” , more “grey puppy”
There’s a big difference though isn’t there between feeling sad or depressed for a reason, (and heaven knows there’s reason enough these days) and just feeling “Eurgh”
If you can identify a reason and can do something about it, that’s fine. But we can’t always change events - only perhaps how we react to them.
Just feeling low could be the result of disturbed or inadequate sleep, harbouring a virus which is yet to appear, lack of sunlight, or of company or stimulus - boredom.
I don’t know the answer, for some people, activity or exercise does it, stimulating the endorphins , while for others just hunkering down with a favourite book or film can do it. Of course a lovely surprise out of the blue usually works - but they are harder to organise!
Treat yourself perhaps, go out for a coffee and slice of cake and if possible try not to overthink it.
Just as we can feel tired physically I’m sure we can also feel mentally tired and need to rest . 
