Sleep difficulties
All respondents in the NHWS were asked: “Thinking of the sleeplessness or difficulty sleeping that you experience, which of the following sleep problems or symptoms do you regularly experience? Please select all that apply”. Respondents then selected the applicable symptoms from a set list. For the purposes of this study, the presence (or absence) of “difficulty falling asleep”, “waking during the night and not being able to get back to sleep”, “waking up several times during the night”, and “poor quality of sleep” were used as outcomes of pain severity as they most closely reflect the insomnia domains of the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-V): sleep onset