Really depends on the type of startup your running. Is it just developing technology? Then that can be easily done nights, weekends (and overnights). But if

Really depends on the type of startup your running. Is it just developing technology? Then that can be easily done nights, weekends (and overnights). But if your startup involves significant operational work, dealing with customers or meeting with clients/partners, then it's almost impossible to do unless you have that work 'outsourced' someway, whether to partners or to other firms, etc.