What would cause me to wake up with a racing heart in the in the middle of the night , causing me me to almost pass out? it happens in my sleep?

Arrhythmia: Next time it happens, try to take your pulse and time it: using a watch (a stop watch would be great) count the number of "beats" for a 15 second period then times that number by 4. Thats your heart rate. A rapid heart rate(>100) while at rest is abnormal. It could be anxiety, caffeine or other medications. Sometimes this has to be evaluated by cardiologist, especially if you get light headed.