I totally agree with this. 2x per week I play a game of soccer after work and then grab dinner with friends. By the time I get home around 9pm I could easily do another stretch of work.

On the days when I don't switch gears, there would be no way I could do focused work for more than an hour or two.

Honestly, if I ever really wanted to do a computer-based startup, I would leave my 9-5 job and do landscaping or something physical during the day. I don't think it's realistic to ask your body to be in front of a computer for 12 hours a day.

+1, in the past where I worked in hospitality, 4 days on 4 off, I could happily spent 3/4 days off building websites.

For the last 2 or 3 years working in agencies, you need a lot of extra willpower to get back on the computer after work.

I work best late at night and early in the morning so just try and fit my side projects in around that time, when I'm most productive.

+ this. I find I'm FAR more productive if I get a decent run in straight after work (where I'm in a PM role) before then getting down to some coding in the evening.

You have to be disciplined though. On the days where I skip the run to get more coding done I invariably actually don't get more done as I have less energy.

Exercise is so so important.