It's pretty common to find yourself dehydrated at the end of a long trip. Air travel is notoriously dehydrating-- between strict TSA rules, a tendency to drink alcohol while flying, and those tiny cups of water handed out by flight attendants, it's difficult to make sure you're getting enough to drink.
According to The Aerospace Medical Association, you should be drinking 8 oz. of water for every hour you fly. This recommendation is pretty straight forward if you're flying from Dubai to Riyadh: the flight is just short of two hours, so you should have two 8 oz. glasses of water.
If it's a longer haul flight, though, you're going to want to drink even more. According to Condé Nast Traveler, "cabin humidity usually teeters around 20 percent, says The Aerospace Medical Association. Other studies suggest it's lower: closer to 10 percent. Regardless, filtered cabin air is usually cool and dry, says Dr. Cowl. "This air tends to pull moisture out of the system. Generally, you get more dehydrated in this type of an environment.""
The more time you spend in the air the more water you need to drink to make up for the dehydrating conditions. If you're also drinking alcohol, you should be mindful to drink one glass of water for every alcoholic drink you consume.
Though it may not seem ideal to be chugging water on a long-haul flight, the inconvenience of having to hop over whoever is sitting in the aisle seat is well worth it: in addition to the benefits of being hydrated, moving around during a flight helps keep your muscles loose and your blood flowing.