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I think people can train themselves to get up at any time, even in the middle of the night, but I have doubts that they'd all be fully alert at that hour even if they did it for months or years. I can train myself to wake up early, but my brain doesn't fully "wake up" until much later in the day. It might be genetic (https://www.sciencedaily.com/releases/2015/05/150514085748.h...) and there are groups looking at drugs to help night owls adapt to the artificial constraints modern society imposes on them.

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I think is a combination of physical activity waking the recruits up and keeping them alert, as well as physical exhaustion helping them fall asleep. I suppose anyone who is chronically unable to stick to that schedule would get weeded out quickly.