I'm tired every Wednesday of the week, because I have band practice and I stay up finishing my homework. Wednesday, my times are 34.xy. and Thursday they get back to normal (about 25.xy).
I usually cube well right before I go to bed. Now, when I wake up in the morning, it's a different story. I stay away from my cubes for an hour or two to give myself time to fully wake up so my times aren't embarrassingly slow.
I am capable of 1 min+ timing when I am tired :fp. Sometimes I do halfway and realise the cross was done wrongly and that screws it. Or maybe I just suck at it.