What Causes Breathlessness While Sleeping?

Hi I started off having occasional feeling of waking in the nights to take a deep breath which felt strained. I now have problems during the day feeling like I need to yawn or take a deep breath to stop air hunger feeling. Have had a chest X-ray, oxygen test, ecg and endoscopy all is normal. I do wake up with a blocked nose. what could this be? Thank you, Frances.