When do red eared sliders hibernate?

Yes, sliders are one type of turtle capable of sleeping under water. They can last for surprisingly long amounts of time but will automatically wake up to rise for air when oxygen levels become low. Even though all are capable, some turtles will non the less choose to sleep with their bodies submerged in water with only their noses or full head out. I've even seen some turtles taking naps totally out of the water. So yes, red eared slider turtles can safely sleep under water if they so choose to.