The 12-on is composed of a 6-hour operator shift, followed by 6 hours of maintenance and training. I think the 12+6 is based on maintaining full awareness over a 6 hour period, and in that sense it basically works. If you eat when you wake up, and after your operator shift, then that also works out to two meals per 18-hour day, and gives you a normal-ish 12 hour gap between your last meal and waking up.