A red eye flight is a flight which takes off from one location late at night, and arrives early the next morning. The slang term for these night flights comes from the idea that many people deplane with reddened eyes caused by lack of sleep. Since most airlines and airports are capable of operating at night, red eye flights are very common in many countries, and frequent fliers often wind up on these flights because of the perceived convenience and reduced expense.
For an airline, red eye flights allow the airline to reposition planes and crew to prepare for the flights it has scheduled for the next day. Since many planes might be moved anyway, the airline figures it might as well break even or perhaps profit by selling tickets on late flights. As an incentive, tickets on a red eye flight may be cheaper than tickets for more coveted daylight flights, and many airlines have a red eye schedule which is designed to be convenient for people like executives, who may want to hit the ground running on the other end, and head directly from a red eye to a meeting or event.
Passengers sometimes find red eye flights convenient as well. Transitioning between time zones, for example, is often easier when people can sleep on a plane and then stay awake all day at the destination, rather than when people arrive in the evening or at night and try to force themselves to sleep. A red eye flight can also sometimes be more enjoyable, since these flights tend to be less crowded, and sometimes people can get upgrades to better seats more easily on a red eye.
When taking a red eye flight, it's a good idea to pack some supplies. A small travel pillow, sleep mask, and a set of ear plugs or noise-canceling headphones can be quite useful for people who want to try sleeping on the plane, as can a soft, lightweight blanket. Grabbing a window seat, if possible, will also increase comfort, since sleepers can lean on the window to sleep. As a general rule, the front of the plane is quieter, since it is far from the engines, and emergency exit and bulkhead rows provide a bit more legroom to stretch out in. Packing water or buying water after security is also advised, as it can be difficult to stay hydrated on a plane.
Many red eye flights run West to East, which means that a few hours of potential sleep could be lost. While it may be tempting to sleep upon arrival to make up for lost time, seasoned travelers do not recommend this. It's better to be active after a red eye flight to accustom yourself to the new time zone and to stretch your body after the long flight. While it's not a good idea to make major plans for an arrival day, wandering around a park, shopping, or making a lunch date can be a great way to stay alert.