We have investigated the scheduling issues of spectrum sensing in the MAC layer in Cognitive Radio (CR) networks. To enhance the efficiency in spectrum reuse by secondary users (SUs), we have proposed an optimal sensing scheduling in terms of finding and adapting optimal sensing-periods of multiple licensed channels in that the overall discovery of spectrum opportunities (i.e., idle portions of channels) can be maximized. We are also interested in building a light-weight spectrum sensor with commercial WLAN devices to shorten the development period of a CR testbed.