While cloud computing is excellent at supporting elastic services that scale up to tens or hundreds of servers, its support for small-scale applications that only sporadically require one VM is lacking. To better support this sporadic usage model, we employ Software Defined Networking (SDN) technology to expose events related to network activity. Specifically, we rely on notifications when switch flow entries are removed or missing to determine resource (in)activity. Our prototype, Sloth, activates virtual machines based on incoming network traffic. Conversely, idle VMs are suspended to conserve resources. We present the design and architecture of our SDN-enabled on-demand resource deployment solution. Our empirical evaluation shows that VMs can be reactivated in less than one second, triggered by SDN events. This on-demand resource activation opens up novel applications for Cloud providers, allowing them to transparently deactivate idle VMs while maintaining connectivity at the same time.