Unlike other animals that hibernate on land, turtles can’t rely on a January thaw to replenish diminishing stores of fat and restore oxygen levels. They have to huddle under mud, sand, sphagnum and other natural shelters to survive the long winter months. Some species, like soft-shell and sliders, can absorb oxygen from the mud, but others, including painted and snapping turtles, must mobilize calcium reserves to buffer acids.
The duration of a turtle’s slumber depends on its age and whether it is diurnal or nocturnal. In captivity, most pet turtles sleep at night and must be kept in a habitat that provides adequate light for them to wake during the day, or their metabolic rate will rise and they’ll overheat.
In the wild, snappers can spend months on the bottom of a pond, sitting in the mud, their metabolism and heart rate slowing to the point of complete inertia. This state is known as brumating, and it enables turtles to conserve energy by using a process of passive gas exchange through their shells.
Some turtles will wedge themselves into tight crevices in rock piles, rip rap and dams for shelter. Others, like Eastern box turtles (Terrapene carolina), prefer to bury themselves in blackberry (Rubus sp.) tangles, which serves two purposes: the sharp thorns dissuade predators and the blackberry provides an important food source. A video taken by an underwater drone of a Northern Map Turtle in winter shows that even with a body temperature of 1 C, turtles can still move around. See around 1:10 in the video.