Guy's Behavior

My brother sleeping without underwear? Why is this a teenage guy thing?

Yesterday I went in brothers room thinking he was up to ask him something but he was asleep and was laying on his stomach with no underwear on ( pyjama shirt on, socks, etc.) and I was obviously horrified and walked out. He's ill today and asleep so my mum went to say goodbye before work but then told me not to go in there so I'm guessing he was doing it again. Why would a teen guy do this? Does have anything to do with I don't know, masterbation or something?
My brother sleeping without underwear? Why is this a teenage guy thing?
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