It's correct, and backed by empirical evidence. Personally, I worked 80+ hours a week for 5 months once, not having a single day off (Weekends included) and by the end of it I was an utterly broken man. Certainly in the last 2 months my skill and ability to actually do what I was setting out to do dropped off a cliff. I can cope with short-term long hours, but long-term, if you want a decent life then 40 hours is an upper bound IMO.
Of course there are people who will claim they can work 60 hours a week and still be productive, which is true, but the question is, can you be AS productive? Ultimately, depending on your realm of work, it might not matter - your total production will increase even if productivity decreases, which could be all that counts. I wouldn't want my doctor or the guy repairing aircraft to be working more than 40 hours a week though.