My 15 month Chihuahua sleeps 10 or more hours a night. Is this normal?

Nothing wrong with that; my Brook will stay in bed with me on my day off for a good 11 hours and snore the whole time. Actually sounding like a barnyard !

Your furbaby is only 15 months old, so not unusual.

Both dogs and cats sleep with one ear tuned to you and the world, so a lot of it is just resting.

Wait until your dog is old - he will sleep practically all the time.