Puppies, just like human babies, sleep a lot. I remember this worrying me too when Scarlet was a new puppy. I would actually wake her up to make sure she was ok (I did this to my human babies too)

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One time when Scarlet was in her regular sleeping spot sleeping I checked on her and her eyes were 1/2 open and looked like they were rolled back in her head. She didnt wake up when I layed my hand on her side and my heart jumped out of my chest. After I gasped and jiggled her she woke up and looked at me like: what is your problem. :o
I just reread your post before clicking submit and it reads that your puppy does this after one minute of play? Now Im concerned. Is this all the time?