Hi,
Fasting blood test is to check for diabetes. The sleep test is to look for aepnoas, where you stop breathing in your sleep. The sleep aepnoa testing really isn't that bad, but you'll feel far from sexy wearing it; you have a band around your waist, one around your chest, a finger probe and a two-pronged probe to go into your nose. There might have been other connections but I can't remember them now.
Basically you strap it on before you go to bed, get up the next morning, take it off and return it to the hospital. It's not exactly comfortable to wear but it doesn't cause any pain. I selotaped the finger probe onto my finger so it didn't fall off overnight because I was doing my test a 100 mile round-trip away from the hospital and didn't want a silly thing like it falling off to muck it up.
Really is nothing to worry about at this stage, but if either test comes back positive you'll qualify under the East of England Specialised Commissioning Group guidelines for obesity surgery.
Regards, Ade.