Among the new specifications of the future 5G wireless communication era, communications between machines reveals new technical challenges to take up. Radio access for IoT, or Wireless Sensor Networks, are among the most popular applications of machine-type communications. They are to be deployed where wired communications are lacking and where the energy distribution network is hardly available. Hence, most of the transmitters are battery-powered without any chance to be changed. In this context, how to save the energy consumption in order to increase the battery lifetime as long as possible? On the physical layer, several new waveforms are competing for the upcoming 5G standardization, all of them being based on filtered multi-carrier modulations. However, the important signal peak power exhibited by multi-carrier modulations has a huge impact on the battery life and should prevent their implementation on the uplink. This talk will show why continuous phase modulations (CPM) can be an interesting alternative to multi-carrier waveforms for IoT and Wireless Sensor Networks. Some issues raised when dealing with CPM will also be presented together with some perspectives on possible solutions.