Sleep disorder. Day at the same time.

I am guy with weak immune system having sinus , thyroid and now recently started facing problem in sleeping. My sleeping was normal till september. But since the last week of September I started sudden awakening around 2 or 3 am in midnight with fast heart beat. Sometimes with normal wake up as I have taken my normal sleep. I consulted with my doctor in medanta hospital so she suggested sleep study test and which is normal report so I dont have sleep apnea problem. But still I wake up around 3 and I wake up  till one hour and after that again I  go for sleep but I can say my overall sleeping time is reducing because earlier I was taking around 7 to 8 hours sleep but now it has been reduced . Again I wake up around 5 :30 or. My doctor told me that I have some kind of stress in my mind. In the beginning of this problem one month ago ,I had some stress related to my health issue and marrige but now I don't have such kind of stress but  It is still awaking me every day at the same time .