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.