I'm 41 and I'm going through exactly the same thing.
This morning I decided to cut my fingernails, went into the kitchen to get the scissors, and forgot why I had gone in there.
I have had every blood test under the sun - all negative.
I have also had an abdominal scan -all fine.
I have been to ENT at the hospital, they have been referred to a Sleep Clinic, where they will test me for, among other things, sleep apnoea, which is where you regulary stop breathing through the night. It is possible to sleep for
eight hours but not feel rested in the morning.