Compensatory drive and sleep-related phantom limb pain: the moderating role of autonomic reserve

Phantom limb pain (PLP) is common after amputation, yet why poor sleep becomes pain in only some individuals is unknown. We hypothesized that a regulatory response is interpretable only against the reserve sustaining it, and thus we tested whether autonomic capacity relative to demand moderates the sleep-pain relationship in 121 amputee adults. Sleep was assessed with the Pittsburgh Sleep Quality Index; PLP, residual limb pain (RLP), and phantom limb sensation (PLS) on 0-10 scales. Heart rate variability was summarized as autonomic reserve (log-transformed SDNN, RMSSD, total and high-frequency power) and autonomic drive (normalized low-frequency power). Poorer sleep was associated with greater PLP and RLP, but not PLS. No single HRV index moderated these associations, but the sleep × reserve × drive interaction did for PLP (b = -0.51, p = 0.026). Poor sleep predicted pain most strongly under high drive with reduced reserve. PLP vulnerability reflects autonomic capacity relative to demand, instead of any single HRV marker alone.