This is an area of active study (how best to sequence sleep-focused vs trauma-focused interventions in PTSD) and I don't think we have all the answers yet. This article provides a nice summary of current understanding here:
Sleep in PTSD: treatment approaches and outcomes .
Here's an excerpt:
"Given that sleep complaints often persist following a course of PTSD-focused interventions and that successful nightmare and insomnia treatments may improve daytime PTSD symptoms, it also is important to understand the potential benefit of sequential or integrated sleep-targeted and PTSD-targeted interventions. A trial examining the efficacy of IRT before CBT for PTSD compared to CBT alone found no superiority of the sequential treatment protocol on PTSD symptoms [27]. However, the group that received supplemental IRT showed greater improvements in sleep symptoms, including nightmare frequency, nightmare-associated distress, and sleep quality. Colvonen et al. [28 ] recently reported on a small pilot study examining the outcomes from an integrated protocol of CBT-I and prolonged exposure (PE), an evidence-based treatment for PTSD, in a sample of Veterans diagnosed with insomnia and PTSD. Following treatment there were clinically significant improvements in PTSD and insomnia severity, with increases in quality of life ratings. The authors posited that providing CBT-I before the start of PE may allow opportunity for sleep consolidation and potentially increase PE’s efficacy. Further investigations are required to determine whether such integrated protocols outperform PTSD-treatment alone, and to determine the optimal timing of when sleep-focused treatment should be introduced during PTSD treatment"