Aquatic turtles may spend hours sleeping on a dry dock or with their head poking out of the water, but they may also sleep underwater for shorter periods of time, coming up to take a breath when necessary. They have a slow metabolism; they can comfortably sleep and rest in water. On average, aquatic and semi-aquatic turtles will need 4 to 7 hours of sleep per day. Many divers have seen green turtles sleeping under ledges in reefs and rocks. Hatchlings typically sleep floating on the surface, and they usually have their front flippers folded back over the top of their backs. They also have adaptations for extracting oxygen from water. Land turtles do not swim like aquatic turtles so they can sleep anytime, anywhere.