Transportation from central Rome to Airport early morning

I'd have your hotel call you a cab. It's a flat fare of € 50,00 from hotels inside the Aurelian Walls (yours will be) to Fiumicino, https://www.adr.it/web/aeroporti-di-roma-en/pax-fco-taxi

Thank you!

A 6 am flight is not going to the US so your flight is basically a domestic flight to some other European city
So arriving at FCO by 4-4:30 for your flight is sufficient
The check in desk won’t be open at 3 am-you might check with the airlines on that
No point getting there too early and just standing around waiting!

Alternative is to stay at an airport hotel the night before
That will get you a bit more sleep and probably a bit less stress.

(I’m always in favor of spending your last night IN Rome though)

I was going to make the same comments that Christine made. That is a local flight so 4.30 to 5 am is more than adequate. The check in desk might not open till then. That a taxi from Rome -- maybe 20 mins at that time of morning.

That is a local flight so 4.30 to 5 am is more than adequate.

Arriving at 5:00 can mean arriving only 30 minutes before the 6:00 flight closes - think Ryanair. FCO is quite large - can take time just walking to your gate once you get through security.

Edit: apologies - earlier statement re KLM Cityhopper was incorrect - boarding was from 30 to 15 mins before departure.

A 6 AM flight is not going to the US so your flight is basically a domestic flight to some other European city

If the flight is to the UK, Ireland or some other non-Schengen zone country, OP would have to go through passport control/exit processing, so less than 2 hours may be cutting it dangerously thin.

... have your hotel call you a cab. It's a flat fare of € 50,00...

Just make sure about the rate and that it is an official cab. We thought we were getting an official cab once, but it turned out to be the desk clerk's brother-in-law.

TC reminded me, years ago I was renting a car at the Avis under the Villa Borghese. I did a recce the day before on foot and they gave me a card and said only use this taxi since they're the only ones who can get into our parking garage.

I asked the desk clerk that night to call this taxi for me and he said sure no problem. Next morning there was the the taxi and ... it was some random different company who couldn't get into the parking garage and left us further away than we we started.

Oh well, the joy of travel.

2 hours prior to departure is the recommended check in time for a European flight, including to the UK or Ireland.
However if you are leaving at 6:00 then I'm pretty confident you are taking the Air France flight to Paris CDG departing at that time, and France is part of the Schengen zone, so two hours will be more than adequate since it will be equivalent to a domestic flight in the US.

Thank you everyone. I really appreciate all your suggestions. I think I got it now!