does it happen right away when you lay down or is it more waking up in the middle of the night short of breath? if it's waking up in the night short of breath it could be sleep apnea. If it happens after you lay down before you fall asleep it could be acid reflux. A lot of times people snack at night and it makes acid in the stomach which creaps up into the lungs and causes them asmatic type attacks or will trigger asthma, this used to happen to me. if you think it might be the 2nd two you can try to watch what you eat at night and take something like pepcid AC or mylanta or something etc. you definitley should talk to a doctor about it though.