I have studied and kept detailed logs of my work for years. When I reach 4-5 hours of "real" work (i.e. deep work) I am mentally exhausted and I consider it a good day.
No one expects you to be 8 hours straight in the zone, no one does. And if you can do so, the more power to you, but be careful not to burn yourself out.
I've worked in a previous position that expected 8 hours of billable work for many of the junior devs, who didn't have meetings or documentation. It was crazy.
Sure, with meetings etc not all 8 hours is going to be deep work, but there's usually more in a workday than that. Meetings, mentoring, etc... But my impression of what the OP is saying isn't really looking at it that way, and is saying 8 hours is too large of a block for work in general, not just deep work.
