I felt alert at the controls as we departed the virtual version of San Francisco International Airport 45 minutes ago. During the takeoff and climb there’s plenty to keep my attention as veteran 777 captain and Boeing pilot Wiley Moore helps me through the procedures of flying a wide-body airliner.

Far from qualified to fly large jets like this, I’m working hard to keep up and make sure everything is done correctly. But once we’re at cruise altitude and the pace slows down, it hits me. I just want to sleep.

“When you were resting [your heart rate] dropped quite a bit,” says Chris Gast, a Boeing statistician pointing to the data after the flight. But it’s not my actual heart rate that captures Gast’s interest — it’s how little my heart rate varies when I’m actually at the controls. Flying rested the day before, there was a much greater change. “The variability in your heart rate is way lower,” Gast says about my fatigued day.