The University of Michigan Health System reports that it is normal for babies to sometimes cry while they are asleep, since they experience the same REM sleep cycle of adults. The University suggests that parents wait to respond to a crying and sleeping infant as this is a normal response to dreams.
GlobalPost explains that nightmares can be particularly intense for infants who have not yet developed their reality testing abilities. This results in the common occurrence of either waking up to cry or crying while asleep. Although GlobalPost explains that only 10 to 15 percent of children have nightmares severe enough to disturb their sleep, parents can alleviate the likelihood of night terrors by reducing the amount of intense and potentially frightening stimuli the child encounters during the day.
In fact, the University of Michigan Health System explains that some children have such intense night terrors, they are actually asleep while they are crying or screaming. Even in this extreme scenario, the University recommends that parents resist the urge to wake the child up and instead leave him alone without disturbing his sleep so he can fall back into the sleep cycle once the short-lived REM stage is over.