Keep Falling Asleep After Eating!

I'm ancient and understand that old people tend to fall asleep more often after eating a meal than perhaps a younger person would. I sit down and the next minute I've drifted off, and not just for a few minutes. Sometimes for half an hour or more, and it's very pleasant, but I do not want to be doing this.

I am pre-diabetic but not on any medication, and try to eat fairly low carb with vegetables and good protein. Avoiding cakes and biscuits.

Any suggestions for stopping this? I have read not to sit down after a meal, but to go for a walk. Honestly I like to sit after I've eaten and this is when it most often occurs. And if I've been out for a few hours, arrive back home tired and sit down, within five minutes or so I've drifted off.

Generally speaking my energies are not as good as they were, so I'd welcome any advice.

Best wishes.