- Dehydration from the low humidity environment of aircraft cabins
- Jet lag
- Insufficient sleep the night before the flight
- Alcohol consumption and/or the increased effect of alcohol at altitude
For example, a flight from the West Coast to the East Coast of the United States might take five hours. If the flight leaves at 10:00 p.m. Pacific Standard Time (06:00 UTC), it will arrive at its destination at 6:00 a.m. Eastern Standard Time (11:00 UTC) the next day, thereby taking the passengers' entire night.
In the United States, Canada, and Australia, red-eye flights fly from the west to the east coast, capitalizing on the timezone changes and the rising sun. But the term can refer to any overnight flight which travels in the opposite direction of the Earth's rotation, i.e., east to west. The term may also be used to refer to many long-distance international flights which are long, even though the aircraft may never travel through a time zone that is in darkness.
In addition to allowing passengers to have a full day at both the departure and destination city and travel by night, red-eye flights operate for the following reasons:
- Repositioning aircraft and flight crew for the following day's schedule
- Increasing the utilization of aircraft in a company's fleet
- Providing additional service to lower cost markets
- Allowing the airline to advertise lower fares to some destinations
- Allowing passengers to connect to morning flights
In the 1930s and 1940s, red-eye flights were not possible, as most airports did not have the equipment necessary to work at night. There are still airports that do not function after certain hours, so red-eye flights can take off only from those airports that are operational after midnight.