Yes, in almost every instance, if an airline provides you with a full set of pajamas in First Class or Business Class on a long-haul flight, you are encouraged to keep them. These garments are intended for single-use from the airline's perspective and are often designed in partnership with luxury brands (like Armani for Etihad or TUMI for Air India) to serve as a high-value souvenir. Many frequent flyers collect these sets to use as loungewear at home. The only notable exceptions are some Asian carriers like ANA or Japan Airlines, which may provide cardigans or "relax wear" that are collected at the end of the flight to be laundered and reused. However, if the sleepwear is presented to you in a sealed bag or if it is a traditional pajama set, it is yours to take. Slippers provided in these cabins are also typically yours to keep, as they are inexpensive and difficult to reuse hygienically.