A memory device having memory cells operates in a normal mode, a first self refresh mode, and a second self refresh mode. The first self refresh mode provides a self refresh operation for retaining data stored in the memory cells without an external command. The time required to return to the normal mode from the first self refresh mode is shorter than a reference time. The second self refresh mode also provides the self refresh operation, but a time required to return to the normal mode from the second self refresh mode is longer than the reference time. The normal mode provides a higher operating voltage to the memory cells than does the second self refresh mode.