My one year old baby cries hysterically during night in his sleep and during day he is absolutely fine . he is generally fit, what is the cause?

Hard to say: Does he waken if you touch him lightly ? If so it's a nightmare. If not, it's likely a night terror, a sleep disturbance similar to sleep walking that occurs in some kids.Most affected kids have poor sleep habits & are sleep deprived. It triggers in the 1st hours of sleep. Creating a relaxing sleep pattern that allows them to move slowly into sleep & get more sleep/night usually corrects this