I think when you begin to work on something, there's the phase where you are "stuck" and need understand the problem well enough to generate an acceptable initi

I think when you begin to work on something, there's the phase where you are "stuck" and need understand the problem well enough to generate an acceptable initial idea to get started. To me, this is the draining part. However, once I have that understanding and can start exploring it, I can easily work for hours; it still requires mental energy and is still draining if I go overboard and don't stop in time, but it's not the soul-crushing kind that being stuck on a problem can be.