When I had a startup, I put a "We own your brain" clause in the employment agreement. However, I also called it to the attention of everybody I hired, and encouraged them to carve out any exception they wanted. Every hire except a couple of pure secretary/office manager types had a carve-out.

That said, I suspect that that's not a common way of doing things. About 30 years ago, I came up with it for a friend who was joining McKinsey, but had published a book based on her PhD thesis. They claimed to have never gotten that request before. But they also didn't object to granting it.