to be honest i never gave too much thought to that. when i wake up and go to turn their lights on my turtles are usually chilin on their smaller basking platform. so they wont be actually on it, they will be in the water but their front arms and head will be resting on the platform out of the water. i doubt that its abnormal or dangerous if they sleep in water. I mean at night while u are sleeping he might wake up and move around and come out of the water, you dont know. also at night while they are sleeping their metabolism slows down immensely so he/she might need a breath or two every couple hours. so yeah pet turtles will definitely come up for some air at night i believe.
Interestingly enough there are turtle species out in the wild who can spend the whole winter underwater without really coming up for a breath WOAH wish i could hold my breath for tht long, dammnnn! haha hope this helps :)