You are not going to get <i>any</i> aggregate stats from YC by posting here. But anecdotally, from a few YC startups I've observed (I've only seen startups run by 20-something founders, so my perception is <i>very very skewed</i>) lived in the office (more accurately, the office was the founders' apartment/house), so there was no distinction between work/life, and the technical founders basically coded as much as possible.