[peteretep]

I have happily programmed for long stretches - refactoring, doing project work, mentoring others. Once you've been a developer a while, you can produce magic without very much mental strain.

Break away from that and do something truly difficult - specification meetings where you're designing a process for huge pieces of hardware that haven't yet been built - studying something complicated and unfamiliar to you - etc - and perhaps you'll find - like me - that you've only got a few hours available to you before your brain turns to mush.