That is absolutely none of your business!
Its weird
Some wear pyjamas, some wear boxers and shirt, some sleep shirtless..why? Because they like it and as long they are fine with it. It depends mainly on temperature. I don't think they sleep shirtless in the base camp of Mt. Everest or in the Antarctic polar station or in winter in unheated rooms.
U can only get a shirtless sim IF hes a boy. To have him shirtless, go to clothing, then click swim-wear or sleep-wear. Find an option that has blank skin. There's ur shirtless sim! (U can have a shirtless girl sim if u count having a bra as no shirt.)
Go to sleep with like boxers
It really depends on the person and the situation. Some boys may choose to be shirtless more often, like at the beach or during sports, while others may prefer to keep their shirt on. It's ultimately a personal preference.
The closest you can come to making your Poptropican shirtless is to: (Girls) Buy the Mythology surfer costume and costumize the upper half,which is like a bikini top (bra). (Boys) The Mythology surfer costume makes your Poptropican completely shirtless.
When i am at home I am shirtless all the time it is more comfortable and some times I just walk around in boxers my family dose not care I even go in in my front yard shirtless and I am only 13. Go shirtless it's more comfortable for guys.
because is somethings natural
Boys can become shirtless (almost) using the characters costumes on Nabooti Island or the Aztec period on Time Tangled island. The only way for girls is to "go furry" on Mythology Island and costumize as the Minotaur!
You don't.
Not everyone wears pajamas or a nightie. Some people sleep naked or only wear knickers or underpants. Therefore, what you wear in bed is entirely up to you.