Arousal is an important protective response during sleep, and arousal deficit is hypothesized to play a role in the etiology of sudden infant death syndrome (SIDS). Because environmental or caretaking factors have been shown to exert powerful effects on susceptibility to SIDS, manipulations that facilitate arousability might be protective against SIDS. All-night laboratory polysomnographic recordings were performed in 20 routinely bedsharing and 15 routinely solitary sleeping healthy breast-feeding Latino infants within the peak age range for SIDS, in both bedsharing (with mother) and solitary sleeping environments. The most important findings revealed by repeated measures analyses of variance were a significant reduction in stage 3/4 sleep and an inverse increase in stage 1/2 sleep on the bedsharing night compared to the solitary night, irrespective of routine sleeping arrangement. Shorter mean duration episodes of stage 3/4 sleep and longer mean stage 1/2 sleep episodes accounted for these differences. Because the arousal threshold is high in the electroencephalographic delta range, by limiting the amount of stage 3/4 sleep, bedsharing should promote infant arousability and might be protective against SIDS. The results also suggest that accepted normative values for infant sleep established in solitary sleeping infants may not be representative of infants raised in social sleeping environments.