I claim the ornamental design for a sleep cap, as shown and described.
FIG. 1 is an isometric view from the right, back side of a sleep cap, as worn on the head;
FIG. 2 is a top plan view thereof, wherein the sleep cap is not being worn;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is an isometric view from the left, top, front side thereof; and,
FIG. 5 is an isometric view from the right, bottom, front side thereof.
The partial human figure depicted by broken lines assists in showing how the claimed design may appear when worn and forms no part of the claimed design. The break lines indicate that any portion of the article beyond what is shown forms no part of the claimed design.