Ambien. Members of SEAL team 6 actually napped on the helicopter ride to take out Bin Laden: https://www.theatlantic.com/international/archive/2012/09/ho...

Humans are amazing in our ability to adapt to the new normal. I was of the same mind of how the heck could you do this, however my body learned when I was in the military to sleep whenever it could under circumstances that I would have never consider possible until it became the new normal. Years latter I had the pleasure of teaching my wife to sky dive. On the ride up to altitude I would close my eyes and nap she was a bundle of nervous energy. She said it freaked her out, I just smiled. The average humans ability to adapt is key to our success as a species. In the end, I did nothing special, I just adapted to survive.