A red-eye flight is any flight departing late at night and arriving early the next morning. The term red-eye, common in North America, derives from the fatigue symptom of having red eyes, which can be caused or aggravated by late-night travel. Inside the US these flights almost exclusively travel from the West to the East. These flights are very low cost since significantly less people acquire them due to the inconvenience of touring so late at evening.
A red-eye flight is much more economical.