Most of the time, it was my hubby would wake up to our son's crying. But if it would take a while before he calms down, I would try to help but usually I can calm him down.
Anyway, speaking of nightmare, I found this article from WikiPedia
A nightmare is a dream which causes a strong unpleasant emotional response from the sleeper, typically fear or horror, being in situations of extreme danger, or the sensations of pain, falling, drowning or death. Such dreams can be related to physical causes such as a high fever, turned faced down on a pillow during sleep (most often in the case of drowning nightmares), or psychological ones such as psychological trauma or stress in the sleeper's life, or can have no apparent cause. If a person has experienced a psychologically traumatic situation in life—for example, a person who may have been captured and tortured—the experience may come back to haunt them in their nightmares. Sleepers may waken in a state of distress and be unable to get back to sleep for some time. Eating before bed, which triggers an increase in the body's metabolism and brain activity, is another potential stimulus for nightmares
Just a knowledge in case anyone wants to know.