Right, but they're not going to do their job very well if they're sleepy. Over time evolution is going to select for the nightwatchers who don't get sleepy and overlook the shadows in the night, leading to humans who are comfortable being awake at night, and humans who are comfortable being awake during the day. The day humans get food, make clothes, etc for the night humans who make sure the farmers, gatherers, etc are safe when they're asleep.

Edit: I wonder if there's a correlation between being ADHD, being a night owl, and gun ownership. ADHD is speculated to be a residual evolutionary trait from when humans were hunter gatherers due to a typically heightened awareness; I wonder how often ADHD humans found themselves as the nightwatch as humans graduated to farming.