The ornamental design for a side sleeper, as shown and described.
FIG. 1 illustrates a top perspective view of a side sleeper according to the present invention;
FIG. 2 shows a front view of the side sleeper, including removable top rail and breathable mesh sides, according to the present invention, the front view and a back view of the side sleeper are identical;
FIG. 3 shows a right side view of the side sleeper, including the removable top rail and the breathable mesh sides, according to the present invention, the right side view and a left side view of the side sleeper are identical;
FIG. 4 depicts a top view of the side sleeper according to the present invention;
FIG. 5 shows a bottom view of the side sleeper according to the present invention; and,
FIG. 6 illustrates a bottom perspective view of the side sleeper according to the present invention.
The broken lines in the drawings illustrate portions of the side sleeper that form no part of the claimed design.