Check out this talk from 37Signals co-founder Jason Fried at TEDx Midwest, specifically at 5:13 when he starts comparing sleep and work:

It got me thinking about other “phase-based” activities, like Tai Chi or Qigong practice. Most productive practice happens when you can allow yourself to move through several stages of settling in and keep a continuous practice thread going for an hour or more. Over the last 6 years of running 3 hour-long classes back-to-back-to-back, multiple times per week, I’ve gotten a pretty good feel for these phases.