One thing I noticed while doing a 40 hour a week software job recently (I'm not working now) is that your ability to work in the evenings and weekends depends on how much cognitive load you go through during those 40 hours.
I've had jobs where the load is moderate, but in this recent one, the employer wanted to extract work from every hour of the 40 hour perior. I was so exhausted, that I couldn't do anything in the evenings, would spend Saturday in a state of haze, and on Sunday finally be able to think straight. But then on Sunday you barely catch up with what you were doing last Sunday. So your progress practically comes to grinding halt.
I've had jobs where the load is moderate, but in this recent one, the employer wanted to extract work from every hour of the 40 hour perior. I was so exhausted, that I couldn't do anything in the evenings, would spend Saturday in a state of haze, and on Sunday finally be able to think straight. But then on Sunday you barely catch up with what you were doing last Sunday. So your progress practically comes to grinding halt.