What Causes Shortness Of Breath While Sleeping After Treated Stroke?

my step-father had a stroke last April. he recovered very well and is fully mobile, and was back at work a month later and driving his car and motorbike and back to playing golf and fishing. The only thing that has me concerned is that when he is asleep he wakes up tryin to catch his breath and sick comes to his mouth, once he gets a glass of water he is ok. Why is this? Its very scary and wakes the whole house up. It happens on a regular basis. Please help.