Personally, I've never had a project that had so little traffic from robots or actual people that I'd expect there to be 4 hours a day with zero hits.

Assuming you have any of the billion spiders out there hit your site more often than once an hour, there's no such thing as sleeping.

I definitely see my hobby dynos go to sleep. I don't try to get those sites indexed on search engines, tho, but (as you might be able to tell from my verbage), I also don't do anything to keep them off. Still, they're asleep. Maybe there's a default robots.txt that tells them not to index, that can be served by a sleeping dyno?