An experienced aviator and traveller, about the physical facts and effects:
This is how your body generally functions, whether you like it or not
When flying, especially during longer flights, your body dehydrates much quicker than what is normally the case. The airplane environment is very dry. The cabin contains less than 20% humidity, often much less, and the cabin air pressure corresponds to approximately 2500 meters in altitude. Your body needs to adapt to low air pressure and to air which is thin and dry as in desert. This leads to dehydration. You can note this by experiencing increased thirst, drowsiness, headache and/or dizziness as well as other jetlag issues. If you are a hard working person with long flights on your schedule, this will affect your ability to perform at business meetings or conference presentation scheduled soon after touchdown.