Pleased to hear you're feeling a little better, Ant
Honestly, if you're feeling blue, I think often it's best just to let it be rather than put pressure on to try and work out whether or not you should be. If you are, then there's a legitimate reason somewhere, even if you can't put your finger on what it is.
That said, sleeping well can make a heck of a difference to this kind of thing as well.
Thanks to @Bamnan and @bugduv for your posts yesterday, btw. Still feeling really mentally fragile (especially as my boss is currently sifting through CVs sat next to me!) but we'll get there.