If by sleep you mean Power Saving Mode(PSM), then you are correct. While in PSM the device stays unreachable to the network and can only initiate communication itself or when it wakes up at designated intervals. It will then be reachable for a certain amount of time based on network and PSM configuration.
An additional power saving scheme for NB-IoT is eDRX, where device will stay connected, but with extended periods where it's not listening to the network.
