In this paper, we present a stepwise adjustment of the constrained application protocol observing period (SACOP) for Internet of Things (IoT) applications using wireless sensor networks (WSNs), which dynamically adjust observing periods depending on the Rx queue status of the constrained application protocol (CoAP) client. The operation of SACOP consists of the following two consecutive phases: overflow alert and observing period adjustment. In the former phase, a client sends a buffer overflow alert when the queue of the client reaches the predefined queue threshold. In the latter phase, the servers change their own observing period level depending on whether or not they receive a buffer overflow alert message. Therefore, SACOP can significantly reduce the number of dropped messages caused by buffer overflow. A simulation showed that SACOP achieved a higher network performance than the legacy approach with regard to the number of dropped messages.