After a couple of days, you start to feel almost euphoric. Definitely less brainfog and more energy.
However, I do think it actually messed up my sleep one night. Late at night, when I'm usually asleep, I had boosts of energy that disrupted my sleep. That was really just one night, around the 48 hour mark, though.
I actually ended this fast last night, a few hours after this post. I was aiming for about 3 days and decided to end it last night instead of today because it just felt right.