I do the same thing with the timer app. I've only worked more than 8 hours when there was a tight deadline. I was surprised by how exhausted I felt on those days. On a normal productive day I get 6 hours, on a day where I feel shitty I get 4 hours. I was actually surprised by the amount of work I get done with only 4 hours of focus per day - no complaints from my boss or anyone else