Long layover at Heathrow

Nothing is open in the airport overnight. My choice would be to spend the night at the Sofitel in Terminal 5, have a good nights sleep and wake up refreshed for the flight to Rome. If doing this you will need to go through immigration and customs.

Thank you! Those were my thoughts. Appreciate the prompt reply!

I wouldn't try to go into the city, or leave the airport at all, especially on the overnight layover. The Sofitel is a good suggestion though expensive. It's connected directly to Terminal 5 so your time from bed to boarding is minimized (but allow at least two hours, you'll have to go through security again. There are other hotels quite close to other terminals, including presumably Terminal 3, maybe cheaper.

It may be tempting to leave the airport on your returning layover if it's daytime, but I'd still advise against it given how long it can take for passport control, transportation, returning, and clearing security again. When I had an 8-hour daytime layover at Heathrow last year I bought a pass to one of the lounges, which made my wait much more comfortable. The free food offset part of the cost.

I have twice used the relatively new Premier Inn that's connected to Terminal 4 by an enclosed walkway. It's about a 10- or 15-minute walk from the terminal, but you are never exposed to the weather. I've gotten extremely good prices at that hotel (under 50 pounds for a single room when booking well in advance), and the rooms are fine if you don't expect a Marriott (or a Sofitel).

Another question: if we do stay at an airport hotel do we need a “short stay” visa?

I assume you are US passport holders. You do not require a visa to enter the UK - you are admitted (or refused entry) just as regular visitors.

Since you arrive at terminal 3 and leave from terminal 5 the Sofitel at terminal 5 is an easy choice. It is connected to the terminal. Right across from the door you exit from terminal 5 to get to the elevators that take you up to the hotel is a Marks & Spencer Simply foods with ready made sandwiches and salads and other foods should you feel peckish. I believe it is open 24 hours.

Once past immirration and customs and in the arrivals hall, turn right and go to near the end of the terminal. M & S will be on your right, the door to the hotel elevator--also the parking garage elevator--is on your left.

This way, in the morning, it's only a few minutes walk to the departure area of terminal 5.

Thanks for everyone ‘s help! Do the inter terminal buses/trains run 24 hrs a day between terminal 3 and 5?

This should help answer your question

https://www.heathrow.com/airport-guide/getting-around-heathrow/travel-between-terminals

Regular service between terminals runs to just after midnght. You shouldn't have a problem.

Some years ago, we had a 10 hr layover at Heathrow. We took the train into London, went to the British Museum ("It's free!!") and had lunch. So far so good.

Our big mistake was getting suckered into the Express (£16/person, for 4 £48) as opposed to the local (£8/person). We spent the extra £24 (at that time about $36) to save 15 minutes total. It probably would have been cheaper to take a taxi with 4. The BM was worth the trip - the Rosetta Stone, the artifacts of Mesopotamia, the Elgin Marbles (why go to Athens, when the most important parts of Greek civilization are in the BM?)

Our case was different than yours - we arrived at about 7 AM, and left at 4 PM. When you are arriving at 10 PM, you have fewer options. That's a really odd arrival time. Mostly flights are overnight. Why do you arrive at night?

Early morning flights from the East coast arrive in the evening local London time.

If you decide to stay in the airport, there is a cheaper place to stay for the night, Yotel: https://www.yotel.com/en/hotels/yotelair-london-heathrow, within the airport terminal.