We introduce a practical green traffic engineering technique in fast reroute (FRR) enabled IP networks which is able to avoid transient routing disruption caused by sleeping-mode reconfigurations during runtime. Following the description on the time-driven "delegate before sleep" operations based on the NotVia FRR platform, we also propose a simple optimization algorithm for selecting sleeping links and determining the sleeping duration according to dynamic traffic behaviors. The aim of the approach is to avoid traffic congestion due to reduced network capabilities during off-peak operation time.