From personal experience, my registrar does hold onto domains for a couple of weeks (maybe a month, I'm not sure). That's to give you another chance if you forget to renew it or forget to pay the bill or whatever.
That may not be the same for other registrars so YMMV, but my registrar does hold onto domains. I don't know if that's the put to sleep thing. And I don't know how it entirely works out, because I've never let a domain expire.
