I was driving back home after spending night at friend's place. It was around 4am, the whole family sleeping in the car. I didn't have any alcohol in my system because I always kept a 0 alcohol policy if I have to drive. I knew the trip well because I was driving it several times a months. A few km before reaching home I realized I had no recollection of having driven past a number of the usual landmarks. Yet I couldn't have possibly been completely asleep as the highway had a number of curves and I would have smashed the guardrails had I been totally unconscious.

So my theory was that I was so tired that my brain wasn't recording anything but I had been conscious enough to actually follow the road.

That was still frightening enough that the next time I just asked to bring inflatable mats and sleep at our friends living room before going home in the morning.