Hello Granny Moss,
Your not alone, in that many people including my wife and i at times after say having had some thing to eat and drink then sitting in a comfortable arm chair get settled as we start to watch a film on the T.V and surprise surprise after a while start to nod off .
this has happened to myself a few times and also to my wife who in our early courting days and even after we were married where when we went to the cinema, manys the time when she would fall asleep ,and after wards she asked me what happened with the film to which i would tell her how the film went and what happened ,
i suppose sitting in the dark in a comfortable seat as the film started off she would nod , it even happens now all those years later when she watches the t.v and once again i have to tell her what happened, so much like many people she saw the start and end of a film or what ever and im filling the middle bits ,
so is it tiredness, ...or getting comfortable ,who knows, well when your retired as many of us are and relax into a state where we nod off when we like, and many of us do, including myself,