It dosen't evolve when it's sleeping, it's happened to me too, and first I thought it was, but they'll never ever evolve in sleep, but sometimes, mostly toddlers to teens, the charachter they evolved to has got another wake up-time, so they simply goes to sleep againYour teen will evolve a 1 day and 2-3 hours from the day your child evolved to a teenager. And yes it is possible for your Tamagotchi to evolve when it's sleeping, I've seen it before.
~Memetchi
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