Sleep sort is kind of like count sort but with added overhead. Both have a complexity of O(n+m) with n being the length and m the value of the max element.
The optimum of O(n * log n) is based on the assumption that the only information about the values is obtained by comparison. If you operate on integer keys I would recommend radix sort. It has a complexity of O(n * w) where w is the length (read logarithm) of the key.