F15 pilots sleep in a warm, dry, non-motion bed after a 15-24 hour mission. Put them out at sea for two months, wet, cold, alone. I don't think you could cram enough drugs into them to keep them awake, I think it's fear.

Remember after that one French guy set a Trans Atlantic record, he fell fast asleap and ran his tri on the rocks while delivering it from the finish line back to France? The post traumatic stress release was too much and he just slept as his boat ran on the rocks, even through the alarms he had set. But I think he was up for about 5 days straight before that.