Ask HN: How do you find the energy for a side project after a 9-5 job?

I've heard/read that some people work 4-5 hours in the evening/night after their full-day job (for a side-project or hustling for their startup). After a day of work, I feel totally exhausted and can't seem to focus on work. Though I can read (books/articles) and walk/exercise a bit. Weekends too go in haze or watching some Netflix. Any suggestions?