I think it is a valid concern, and these sort of working days the "system" tries to impose upon many people. I've certainly had a fair share of 18 hour days.
Thankfully I'm now in the happy position of having enough clout to tell the "system" to get stuffed and, quite frankly, don't trust myself not to make mistakes if I'm working longer than a 12 hour day routinely - and that's bad enough.
The line I take has always been "Sorry, I've been on the job too long, I'm worried I might make a safety related mistake and need a break here". It's a line that, used appropriately is very hard to argue with - just like the Cap'n who wouldn't accept an aircraft with a u/s generator. I sent a whole section home for a day once doing a job for Saab, on the ground that they'd been all working 15-18 hour days for 6 days and needed the 7th off. Believe it or not, I got complaints about the loss of overtime from my own staff, but Saab were understanding and didn't kick up a fuss.
G