Why has my baby started waking up hysterical at night?

New stage/habits: This may be prior to your baby reaching a new developmental stage, hunger, a need for soothing, and/or may be related to an illness (fever). Another cause is a night terror, an episode during which your baby may be screaming but is not really awake. Just check on him to make sure he's ok but do not intervene. You may change the terror by slightly waking him just before the time it occurs nightly.

Look up "night...: ...Terrors": http://www.Nlm.Nih.Gov/medlineplus/ency/article/000809.Htm if it sounds like what you have, it's normal.

Depends on pattern: Night events for baby can include: awakenings, terrors, nightmares &separation anxiety. Nightmares occur in a light sleep state so baby is aware of you immediately if you check them. Night terrors occur in deep sleep & look like a crying fit or struggle.They are not aware of you and don't waken to light touch. Occasionally some show extra separation fear at points in development.