There's likely a shift differential, too. I'm not a nurse (yet), just a CNA, and I get an extra $1.90/hr working overnight (7p-7a) shifts at a hospital. I'm not sure what the nurses make, but their base pay is roughly 3x mine, so I assume their differential is proportional. I also get $0.95/hr extra on weekends. This is why I work nights, and most weekends. :)
So you have her base pay, a shift diff, overtime, premiums for working during periods of short staffing... it can add up really quickly.
eta: Just went on our staffing website and signed up for some extra shifts. This was a motivating read, thanks!