As others have said, working 12 hour days is likely your problem. You are probably naturally compensating to not get worn out by instead doing sporadic, low pressure, tasks.

Think of it in reverse. What would a day look like if you really focused on one thing. Presumably you would be worn out if not after 4 hours at least after 8. Now you would have another 4 to go. That isn't sustainable. Doing all sorts of random things that doesn't need to produce a result probably is.