Contrary to popular perception, studies show that parent-infant bedsharing is not uncommon in American society. A belief that bedsharing with infants negatively impacts the quality of adult sleep also appears wide-spread. This has not been substantiated, however, because the few studies that have measured the impact of bedsharing on adult sleep examined only bedsharing with another adult. In the present study, laboratory polysomnography was performed in 20 routinely bedsharing and 15 routinely solitary-sleeping, breastfeeding, Latino mother-infant pairs comparing the mothers' sleep when bedsharing to solitary-sleeping nights. Infants were 11-15 weeks old at the time. Irrespective of routine sleeping arrangement, mothers' total sleep time was not decreased on the bedsharing night compared to the solitary night. Across the two groups, percent Stage 3-4 sleep (of total sleep time) was significantly reduced on the bedsharing night but only by 3.9%, while Stage 1-2 sleep was increased 3.7%. Episodes of both Stages 3-4 and 1-2 were significantly shorter. The amount of rapid eye movement (REM) sleep was unaffected. Overall, arousal frequency was significantly increased by 3.6 hour-1. As the increase in arousal frequency was stage specific, it could account for the pattern of stage changes. Nocturnal wakefulness was not increased, however, because awakenings were of shorter duration. these effects of bedsharing did not habituate with routine bedsharing because they were not diminished in the routinely bedsharing mothers compared to the routinely solitary-sleeping mothers. We find that the impact of bedsharing on maternal sleep is modest and somewhat different from the reported impact of sleeping with another adult. From the infant's standpoint, the effects on maternal sleep are adaptive to the extent that opportunities to monitor the infant's status are enhanced. The mother's caregiver role is likely germane to differential effects on sleep of bedsharing with an infant versus another adult.