My experiences suggest the answer is: possibly. Or perhaps more accurately sometimes.
A lot depends on the field you're in. I studied for a PhD in life science, where a lot of time at the lab bench was required. This is skilled work, but it's not "creative" nor does it involve much mental effort. So it's certainly possible to be productive at it for longer than four hour stretches.
In addition I often had to go in at the weekends to observe the results of my experiments. Cells don't grow to a useful 9-5 weekday schedule, unfortunately! I imagine other areas of science will impose similar time pressures.
I treated my PhD as though it were a job. Although I did work longer than 8 hours a day and I did work weekends when necessary, I viewed this as an annoying imposition and tried to minimize it. Other students and postdocs in the lab did longer hours and were more productive.
When it came time to write up my thesis, I discovred I simply did not have enough material to make it worthwhile. Ultimately I was forced to apply for a lesser research degree (an MPhil) and when it came to the crunch, I was not even able to obtain that with the evidence I'd gathered. Part of this is unquestionably down to lack of bench-hours.
I cannot speak about non-practical subjects, but even there I would imagine the amount of reading, learning and documentary research required would be significant, and would not involve creative mental effort. But my experience suggests that while ten hours a day, seven days a week is likely excessive, a successful research degree does involve time and effort well beyond that required for a regular highly-skilled job.