Good hotel. Better mattress than yours, probably. And you lie there half awake all night like something's about to happen.
It's common enough that sleep researchers had to work around it: when they study people in a lab, they routinely throw away the first night's data, because nobody sleeps normally on it. They call it the first-night effect.
Half of you stays on watch
One side of your brain sleeps lighter than the other, on purpose.
When researchers looked at what was actually happening, they found something strange: the two halves of the brain weren't sleeping equally. One hemisphere stayed in shallower sleep than the other on that first night in an unfamiliar room.
And it wasn't idling. The more wakeful side responded more strongly to unexpected sounds, and was quicker to pull the whole system awake. It was listening.
Ducks do this, and they're better at it
Plenty of animals do this properly — ducks and dolphins can shut down one hemisphere at a time and genuinely sleep with one eye open, one half resting while the other keeps watch.
What humans have looks like a faint version of the same trick. Not a full split, just a lopsidedness: enough to keep something on duty in a place you don't know yet.
Why the second night is fine
Because the room is no longer unfamiliar. Nothing bad happened, so the watch stands down on its own. You don't have to do anything to make that happen — which is worth knowing, because the standard reaction to a bad first night is to lie there dreading the second one, and that dread is a much bigger problem than the effect itself.
What actually helps
Make the room less new. Your own pillowcase, your own noise, the same thing you always do before bed in the same order. You're not making the room comfortable, you're making it recognisable — that's the bit your brain is checking.
And if you have something that matters on day two of a trip, plan around the first night rather than fighting it. It's a known effect with a known expiry date.