A memory system may include: a nonvolatile memory device; a volatile memory device to which power is suspended in the sleep mode; and a controller configured to temporarily store internal data in the volatile memory device, the internal data being generated during processing of an operation of the nonvolatile memory device. When a sleep command is received by the memory system from a host, the controller may output the internal data stored in the volatile memory device to the host in response to the sleep command, and then may transfer an acknowledgement for an entry into the sleep mode to the host and enter the sleep mode.