If someone told you he able to work 12 hours per day that would be a lie. Actually, it depends on how you manage the work time for one day.
If I told you I able to work for 12 hours, that would be a truth. How come? Well, I don’t work straight 12 hours per day.
So, how do I manage 12 hours of work per day? Its not that hard, I just need to chunked my work time to 3 parts, like working for 4 hours in the morning, 4 hours around afternoon, and 4 hours at night. The advantage of doing this I can slip my personal schedules in between, like after 4 hours working, I’m doing another things like clean up desk or even reading interesting books.