Losing My Edge?

I have begun to realise something monumental, lately.
Something that could be significant, although how, I’m not sure.
Not sure how to explain it either, but I’ll try.
For quite a while now, I have been unable to do much of anything after 9pm. The brain just refused, and I put it down to tiredness and thought nothing of it.
But just lately, this has been getting earlier and earlier, and the funny thing is, I don’t feel particularly tired. It feels as though someone throws a switch, leaving me incapable of rational thought. Any kind of thought, come to think of it!
After 6pm these days, I can still function, watch TV or read a book, but don’t ask me about any of it. I am beginning to feel like an evening robot.
I’m not unduly worried. There is no pain or confusion, and the brain works perfectly well all day. I say perfectly well, but do still have the odd weird thing going on, like trying to put the milk in the bin. And I repeatedly forget why I went upstairs, only to remember when I come back down again.
Am I wearing my brain out these days, a natural thing, or is there something more serious going on? No one in our family has ever developed dementia or Alzheimer’s, so I have no clear idea of what to look for. I don’t feel ill or anything, and it doesn’t bother me that after 6pm I can’t think straight.
I am 72 years old, so it is conceivable that something could have decided to malfunction. It happens to everything else around me.  The PC, keyboard, toaster, TV and just yesterday the microwave. So why not me too?
And no, I will not be bothering my doctor with it. Not a lot he can do about it anyway, right?