Why do you feel dehydrated when you wake up from sleeping?

Answer 1

It depends. There are many things that may cause dehydration during sleep. Sleeping with open mouth will dehydrate you, for example. Another example is that there is no water intake during your sleep, however, your kidney continuously creates urine to remove excess salt and urea. When no water is contributed to this from the external source, your body uses the water that's already in your system, from cells, blood, etc. This stimulates thirst.