[gst]

It all depends on the type of your work.

While I did my PhD I've often worked multiple days without a real sleep in between, because some problems were so interesting to me. At those times I would wake up in the night, think about how I can more effectively implement something, and then spend the remainder of the night hacking on the problem. I didn't even really consider this as work, but more as side-project for which I have infinite time and for which I'm getting paid for.

On the other hand, there are also some dull works you have to do as a research assistant, such as writing project reports, so that the funding agency continues to send you money. When working on those kinds of things it often took less than 8 hours until brain fog set in and forced me to work on more interesting things. If writing those reports would have been a full-time job I'm sure I wouldn't even have handled standard 9-to-5 work without going crazy.