Michael Phelps isn't like most people, so it stands to reasons his pajamas aren't like most people's. There are no fuzzy bunny slippers and sheep-print flannel pjs for Phelps. He wears a high-tech compression suit from Under Armour.
The Recharge Energy Suit is a full-body suit designed to help the body recover from fatigue after training.
Under Armour claims the super tight compression fit pushes out the water damaged muscle fibers take on when sore, leading to a faster recovery. This is also probably as close as you'll get to knowing what it feels like to be swallowed by a snake.
Like other performance fabrics, the Recharge suit wicks moisture and features anti-odor technology. In case you were wondering, it also has a working fly for convenience.
Michael Phelps has been endorsing Under Armour for a couple of years, which is an interesting choice for someone who is usually shirtless in the pool. In an interview with Yahoo Sports, Phelps says he wears the suit after he works, after a meet, and when he sleeps.
The men's Recharge suit is currently sold out, but the women's version is available for $129.99. I might also suggest using it as the basis for a superhero costume. It looks a bit like what would happen if Batman's costume melded with Spider-Man's outfit.