ETRI DONG-EUI Univ. This document describes the use cases of communication considering sleepy nodes and the problems of connecting to sleepy nodes in constrained node networks. The use cases of communications between sleepy nodes and non-sleepy nodes are classified by the end-to-end communication and the network topology. The adopt of power saving in constrained nodes raises compelling problems in network layer/ transport/application layer. In this document, problems of each layer in a sleepy node are described.