I’ve heard that some planes are designed to help with jetlag. Is that true?

Sounds like you are referring to the Boeing 787 and the Airbus 320. These two new-ish aircraft are pressurized to only 6,000 feet and have humidity levels above 35%, which is easier on the human body. Compare that with most other commercial aircraft which are pressurized to 8,000 feet and have less than 20% humidity, which tends to enhance the dehydrating effects of flying. Maintaining better hydration not only speeds recovery from jetlag, but also reduces one of the risk factors for deep vein thrombosis (DVT).