A layover long enough to leave the airport is a different planning problem from a short connection. The decision about where to sleep is governed by transfer time and security, not by the attractiveness of the destination.
The usable window is smaller than the gap
A published connection time includes deplaning, immigration if the itinerary crosses a border, baggage retrieval where required, and the return through screening and back to a gate.
Those steps commonly consume two to three hours at a large airport, and they land at the start and end of the gap rather than being spread through it.
Subtracting them from the total is the first calculation. What remains is the only time actually available for rest or for a trip into town.
Checked baggage decides whether leaving is possible
On a single ticket, bags are often checked through, which means they are inaccessible during the layover and the traveler leaves the airport with only a carry-on.
On separate tickets, bags usually have to be collected and rechecked, which forces a landside exit and creates a storage problem for the intervening hours.
Airport left-luggage counters exist for exactly this, and their opening hours are the constraint that catches people out on an overnight gap.
Airside options trade space for time
Staying inside security removes the largest variable, since there is no reclearance and no transfer. The cost is a narrower set of facilities and less room.
Lounges with rest areas, shower rooms and small sleeping cabins occupy this niche. They are sold in hourly blocks because the demand is measured in hours.
The practical ceiling is comfort rather than availability. Few travelers rest well airside for a full night, which is what pushes longer gaps outward.
Airport hotels sit at the middle distance
A hotel on airport property or on a short shuttle loop restores a real bed and a shower while keeping the return trip short and predictable.
The trade is one full pass through screening on the way back, plus the shuttle cycle, which is why the frequency of that shuttle matters more than the room does.
For gaps of roughly eight to fourteen hours this is usually the shortest total travel time to a bed, which is the metric that matters at two in the morning.
City stays need a visa check and a margin
Leaving the airport means clearing entry requirements, and entry rules for a transit passenger differ by country and by nationality and change over time.
Some countries operate transit-without-visa arrangements and some issue short transit permits, but the terms are specific and worth confirming with the relevant authority rather than assumed.
Where a city stay is possible, the useful rule is to book near the airport train line rather than near the attractions, because the return leg is the one with a deadline.