Thirteen women in late stages of pregnancy underwent a polysomnographic study. Eight women (61%) complained of mild nocturnal back pain or back discomfort. Five women (39%) did not complain of nocturnal back pain. The two groups did not differ in total bed time, total sleep time, sleep latency, and wake after sleep onset (WASO). A significant decrease in rapid eye movement (REM) sleep and an increase in stage 2 were observed in the pain group. The same group had a statistically significant decrease in the basal O2 saturation level. The pain group also spent a longer time sleeping in the supine position. We hypothesize that a prolonged stay in the supine position leads to obstruction of the vena cava. In the presence of inadequate collateral circulation, increased pressure and venostasis in combination with a decrease in basal oxygen saturation may lead to hypoxemia, compromise the metabolic supply of the neural structures, and result in pain. It appears, therefore, that the vascular system plays an important role in the pathogenesis of pain. The role played by the disturbed sleep architecture in the production of pain remains to be established. It is possible that the changes observed in sleep architecture result from pain rather than contribute to pain production.