fruitsalad - Chances are you are being tested for sleep apnoea, a condition where you stop breathing for a couple of seconds in the night - it means your heart kicks up a gear to re-start your breathing, and over a long period of time, it can cause strain on your heart.
If you have the condition, you may have to wear a C-PAP device, which pumps air into your nose and mouth as you sleep, to keep your airways open.
Be warned, diagnosed sleep apnoea is a condition which can affect your driving, and failure to accept and use treatment may result in your licence being revoked, so tread carefully.