The air inside a plane in flight is dry. How dry? It is as dry as a dry summer day in a desert environment. This is often an issue inexperienced or casual travellers forget. Minor dehydration worsens jet lag symptoms but in severe cases can cause fainting and light headedness.

Although having one alcoholic or caffeinated beverage is acceptable, remember that these tend to increase the level of dehydration so limit their consumption. 

How to prevent dehydration due to dry airplane air

Remember that dehydration impacts your body inside and out. 

To combat internal dehydration drink plenty of water (typically 8oz for every hour of flight). Limit consumption of coffee, tea, soda, alcohol and energy drinks. If you do drink one of these, increase your water intake to compensate.

To combat external dehydration, apply a good long lasting body moisturizer before and after the flight. If you wear contact lenses, make sure you hydrate them every couple of hours of flight. Wear comfortable natural material clothing.