If you wake up in the middle of the night thirsty then it could mean that you're not getting enough fluids during the day to stay hydrated.
Waking up in the middle of the night is usually a sign of dehydration so I would recommend upping your intake of water and fluids during the day to stay hydrated.
You might want to drink a glass of water an hour before going to bed or keep a glass of water near your bed so you can easily drink a glass of water during the night without having to leave bed.
Another possible reason you wake up in the middle of the night thirsty is due to drooling which could be drying out your mouth.
Another possible reason you could be waking up thirsty besides drooling and dehydration is the onset of diabetes as diabetes can cause you to wake up thirsty in the middle of the night as well.
So if you continue to wake up in the middle of the night thirsty or it has just suddenly started happening then seeing a doctor to get tested by diabetes would be a good idea.