It stops working.
My body did the same thing this last week. I've been going 100 miles an hour for over a month. My day job was busy, my writing life was busy, and my home stuff was crazy.
As soon as I closed out my last Facebook party for Killer Party's release, my nose started running. I thought maybe it might just go away, but no luck. I was sick all during RWA Nat'ls.
I'm still fighting the tiredness, but I worked out this afternoon, so I'm hoping I'm on the other side of this mess.