The first time I experienced M.S. fatigue, I was living on the Peninsula in the S.F. Bay Area. It was exhaustion like I'd never known. I remember trying to fold my clean laundry in a laundromat and having to stop and rest after one or two pieces at a time.
My arms felt like lead, my legs felt like they were shuffling through quicksand and my brain was so, so foggy. And sleeping didn't help although getting out of bed each morning was a battle. The exacerbation lasted a few weeks.
As with all my symptoms, it seems that once I've had it for the first time, the subsequent times I get it don't seem as severe. In reality, I suspect that they are as severe but that I become desensitized to them. Or maybe it's just never as scary ever after.
In any case, I think I am in the throes of an exacerbation, and the current heat wave isn't helping.