IMO this would be unreasonable. It's before the morning shift arrives at most hotels and before breakfast starts. Informally, I think of the line as that you cannot arrive at a time that people would go to bed but you can arrive when people are up and going off to start their day. With an 8 am arrival, of course someone could try to take a nap, but many would shower and head to a business appointment. So an 8 am check in seems reasonable if your room is ready. At 5 am, the instinct would be to crawl into bed for a very short night, so IMO that would not be reasonable.

Personally I would be embarrassed to try to check in too early if I hadn't booked a room for the previous night unless there were some truly extenuating circumstance such as a flight diversion where a room for the previous night wasn't available.

However, I suspect that in practice hotels in some locations, including 24/7 international airports, almost expect people to come and go at all hours and they have housekeepers on duty around the clock. In these places, it's not a big deal and they implicitly do "24 hour check in" and approve very late check outs whenever they can.

Obviously YMMV.