What the Experts Say. The experts at the American Cleaning Institute (ACI) have detailed a helpful guide for how often we should wash our clothes. For pajamas, they suggest washing them after three or four wears. That's about two pairs of pajamas a week.

If you are gifted a pair of pajamas in First or Business Class on an international flight, you can keep them forever. Unlike blankets, pillows, or headphones—which are the property of the airline and must stay on the aircraft—pajamas are considered a "disposable" amenity for hygiene reasons. Once a passenger has worn them, the airline cannot wash and reuse them for another guest. Many frequent flyers build up entire collections of sleepwear from airlines like Emirates, Qatar Airways, or United (Polaris). They are yours to take home as a souvenir of your flight. In fact, many people wear them as loungewear at home for years. The only exception is if you are "borrowing" a cardigan or a wrap provided by certain Japanese airlines (like ANA or JAL) for use during the flight only; these are typically collected by the crew before landing, so always check if the item is a gift or a loan.