I remember driving back to Scotland from a holiday in Harrogate with a friend. (This was in 2018 and I was driving my Golf.) I was fine when we left Consett after having lunch, and continued to drive. But we soon hit trouble. The A68 was closed and we had to take the A69 across the Pennines to get to the M74 - just as a fairly heavy fog came down. It was a strain.
I must have been OK when we passed Gretna, because we didn't stop there, but soon afterwards I was suddenly hit by really marked fatigue. I must have done something that was obvious, because my friend said, are you all right? I said no, I'm just looking for a place to get off this motorway and get a coffee, and after we've done that, you're driving.
I managed to get to Annandale Water without collapsing over the steering wheel, but it was quite scary. I wondered afterwards if I should have stopped on the hard shoulder then and there and switched drivers. At least I did have a spare driver with me, who was insured to drive my car. That's not usually the case. If I'd been alone, how long would I have to have stayed at Annandale Water before it would have been safe to go on?