They sleep very much like humans, anonymous. That is to say, they don’t curl up but rather lie down wherever they can. However, they don’t necessarily need special beds. Oftentimes, they’re content enough to sleep on whatever soft surface you have at the ready, including a couch, a cot, blankets on the floor, the ground itself, trees (in the case of the more … unusual sorts of fighting- and grass-types), or in very rare cases, your bed. Of course, many trainers still like to offer a simple bed to their pokémon anyway, although this isn’t always feasible for every trainer.
As a separate note, regardless of what less scrupulous blogs may have you believe, pokémon such as gardevoir, lopunny, and so forth are not the pokémon I was referring to when I said some human-like pokémon enjoy sleeping in the same bed as their trainers. (In fact, gardevoir especially prefer sleeping spaces as far from their trainers as possible, due to their inherent psychic abilities.) Rather, I was referring to sawk, throh, mr. mime, elgyem and beheeyem, and members of the machop line (but especially machamp).
Furthermore, I would not advise sharing a bed with a machamp. I will let your imaginations come up with the reason why.