Sleep states are an important and well-understood feature of modern server and desktop CPUs that enable significant power savings during idle and partial load scenarios. Making proper decisions about how to use this feature remains a major challenge for operating systems since it requires a trade-off between potential energy-savings and performance penalties for long and short phases of inactivity, respectively. In this paper we analyze the default behavior of the Linux kernel in this regard and identify weaknesses of certain default assumptions. We derive pathological patterns that trigger these weaknesses and lead to ‘Powernightmares’ during which power-saving sleep states are used insufficiently. Our analysis of a workstation and a large supercomputer reveals that these scenarios are relevant on real-life systems in default configuration. We present a methodology to analyze these effects in detail despite their inherent nature of being hardly observable. Finally, we present a concept to mitigate these problems and reclaim lost power saving opportunities.