Want to check in to your hotel early? Good luck.

It’s a travel math problem that doesn’t always work out: Your flight lands early in the morning, but the hotel check-in time isn’t until 3 or 4 p.m.

Sure, you can veg out in the lobby or leave your bags with the bell desk and explore. But if what you really want to do is shower and nap, you’ll have to pray to the early check-in gods or, in some cases, pay an extra fee for those additional hours.

And don’t forget that check-out is typically at 11 a.m., in case you had plans on your final day. Staying for just one day? Enjoy your 20 hours at the hotel.

The often-inconvenient times — which one operator called “an ancient hotel convention” — are set by hotels to make sure guests vacate their rooms with enough time to prepare them for the next round.

“It’s a function of housekeeping,” said Gary Vallen, a hospitality consultant and co-author of the textbook “Check-In Check-Out: Managing Hotel Operations.” “Getting a room checked out, going and stripping that room, cleaning the room, getting it brought back online, having an inspector from the housekeeping department check it ... takes you a little while.”

Knock, knock. Who’s there? Not housekeeping.

For the average traveler, there are some ways to solve the issue of needing a hotel room before or after one is supposed to be available. It never hurts to ask if early check-in or late check-out is available for free. Some hotels might offer those benefits for a fee; hotel chain citizenM, for example, says check-in is guaranteed at 2 p.m. but available earlier for an extra charge at certain properties in Paris, Amsterdam and Taipei.