Turtles regulate their internal body temperature by basking in the sun. You may often see them stretching their legs on the rocks in the sun. Unlike mammals, reptiles are cold-blooded. As such, their body temperature is determined solely by the temperature of the surrounding environment. In other words, they are hot when their environment is hot and cold when their environment is cold.
During colder months, turtles will stop eating and become inactive. This is called brumation and is a reptile form of hibernation.