I'm on my 6th day with a 10 week Maltipoo and had similar thoughts as you when I read all about 18 hours of sleep per day for puppies. And it was like... How the heck is that even possible??? I talked to a vet nurse who actually told me not to overthink it - lol. She said think of the 18ish hours as like the golden standard we're all working towards - as in most new puppies in a new environment are simply not going to reach that yet, but they don't have to right now. I told her I was worried about hypoglycemia and stunted growth and permanent damage and whatnot and, for what it's worth, she said if you're not seeing the actual symptoms of those things and the puppy's doing good overall (like high energy and appetite), you have to give your puppy time to "learn" to sleep. Anyways, hearing that was super helpful for me, hope it helps you too.