I have experienced once.But only once ,Im also pretty wierd that why it dosent happen again.At about one year ago ,in the middle night,1 AM maybe.There was no omen when i woke up,i just felt a strong sense of fear flocking in my mind,i dont know why ,i tried to control it ,But actually i failed,And i began to shake,The speed of heartbeat is increase pretty fast.A strong feeling just came to my mind.I was dying,especailly when it reached the peak(just like orgasm actually).But after that point ,i felt a little bit better,my breath seemed not as rapid as beginning.After this panic attack ,i felt high level of awakeness,and couldnt sleep until about 3 AM.Thats the whole process of this experience.Dose anyone who have the same experience?im just curious about that,By the way,it seems that there is no stressor for me at that day,so my assumptive explaining for that is maybe something imbalance for physiology caused that.