I think you'll need to provide a bit more information before anybody can help you! For example, did you book this as a package? What was the reason for arriving at that time? Was your flight delayed or was this always the expected time of arrival?

If the room was ready for you to occupy ,then yes,it counts as first night - it was probably cleaned at some stage during the previous day.

We originally booked an early morning flight but it got changed to a night flight as they amalgamated 2 flights into one. We arrived at about 3.30am and our return flight is also a night flight but we have to vacate our room at 11am to be picked up at about 11pm for our flight home. It doesn't seem right that we have to pay to keep our room on when we didn't choose to change the flight and we didn't get into our room until approx. 4am.

Sorry, but in that case, yes, it does count as your first night - the alternative would have been to wait until the afternoon before you got into your room. Are you actually still in resort? If travelling on the night flight would have been cheaper than the flight you originally booked then I would suggest that the best you could hope for would be to be refunded the difference. Hopefully that would compensate you for what you will have to pay keep the room on? Or could you contact your TO to get them to arrange for the hotel to waive the extra charge for you?

Thank you for your reply.
I will certainly be complaining when we get home as we have just been informed that our flight home tomorrow night has been pushed back again!!! We are waiting to find out if there is a room we can pay to use but it seems unlikely. Therefore, we will have no room for over 12.5 hrs tomorrow.
Thomson have said they are sorry for the inconvenience but that's it.