From 8 to 20 hours watchstanding, you're basically not watchstanding. It went like this:

    Hour 1: Fine

    Hour 2: Walk around a bit, then fine

    Hour 3-5: Jesus, I never should have joined the Navy

    Hour 6: I want to pull out my 9mm and use it on myself

    Hours past 6: I am functionally useless

Some jobs that aren't knowledge-jobs are hard on your mind, too. The reason security/watchstanding is so hard is that you have to be alert at all times while doing, effectively, jack squat. It isn't a much different level of alertness than I am in while programming, now that I think about it carefully.