While I wouldn't necessarily classify today as a 'bad' day, it definitely isn't a 'good' day. My first thought was it felt like my body was in a vat of treacle as I have to struggle through every movement. However, this isn't exactly right, as while my body is heavy, it can move like usual. Instead of my body moving through treacle, it's my mind that has to use the effort it would need if I was stuck in treacle (just to clarify, this is all based on assumption as I've never actually been stuck in a vat of treacle!); every movement requires almost all my concentration and mental energy regardless of how it physically feels.
I don't have these days too often, and yet when I have them I find I am more inspired than usual. The prolonged stillness and struggle to even form thoughts somehow leaves space for the really interesting stuff to rise in my mind. Of course, I then have to decide whether it's worth the pain and struggle of writing it down (I am trying to get into using my dictaphone but I'm really bad at remembering to check it afterwards) as otherwise it will quickly sink, being swallowed by the treacle encasing my mind.