The last time I felt that was during an extended bicycle tour in the Adirondacks in the mid-70's. It was bad -- we'd get off the bikes so numb that we didn't realize our bladders were full to bursting. As soon as circulation returned, we had 15 seconds or so to find a bathroom or a bush.
I haven't felt that since. Differences between now and then?
That tour featured extended periods - two or more hours at a stretch - in the saddle. I now know to vary my position, and even stand for short stretches.
You may need to check and adjust your seat height - it might be too high.
Saddles back then were instruments of torture; they're much better now.
Have you tried different seats? It took me four tries before I settled on my current seat - a Salsa Zona Tres.
If I don't vary my hand positions, I'll get numbness in the hands and wrists. Move around - ride on the hoods, move to the drops, then move to the top of the bars. I rarely keep the same hand position for more than 5 minutes at a time.