Some good advice on here. In my experience the hotels do a night audit between 5 and 6am. I've found that it's best to wait until 6 if you want to request an early check-in. I have never planned an early check-in ahead of time. It has always been a last minute situation I found myself in. If you have status with the hotel and they have rooms available then they usually accommodate you. Some hotels have night time housekeeping. Usually this is because they have RR crews or flight crews staying there. If you know this ahead of time it increases your success rate.