Usually, no matter how tired I am, I can somehow muster the adrenalin to get through a gig. The drive home can be difficult, but the gig itself is usually no problem.
However, a couple of months ago, I had to do a few gigs while fighting mononucleosis. I'd pretty much be sleeping all the time. I'd wake up to get ready for the gig, sleep on the drive to the gig (it's the first time in ages where I couldn't drive myself), sleep on our breaks, and sleep on the way home. By then, it would be bedtime.
I was a total zombie. Thinking back, I really have no clue how I got through it. I'm glad I'm over it though.
Later...