IIRC, after around 60 hours of coding, developers start introducing bugs faster than they fix them. This obviously varies from person to person, but yeah, you can do negative work pretty easily in software by not thinking. It took me a long time to learn to just quit for the day and get a good night's sleep when i'm struggling with something for an excessively long time.
But there IS certainly a gap between 40-hr work-week, and writing terrible buggy code because you're burnt out.
Putting in a few longer days a few times a month, you won't burn out, you will get more done, and yes, it is harder on you.