Why this is worth the effort of asking
One more finding, and it is the reason a sleep course sits next to the courses on anxiety and low mood rather than off on its own.
What the research says
Scott AJ, Webb TL, Martyn-St James M, Rowse G, Weich S (2021). Improving sleep quality leads to better mental health: a meta-analysis of randomised controlled trials. Sleep Medicine Reviews, 60, 101556 doi
Pooled randomised trials in which sleep was improved, and found that mental health improved alongside it, with larger improvements in sleep going together with larger improvements in mood and anxiety. No figure is quoted here because none was read off the paper. The direction is the useful part: treating the nights is not a side project.