Hi Andrew, I was advised by a doctor to drink 2 litres of water in 24 hrs and ensure sufficient salt intake (which I think is generally about 6g (2.4g sodium, including salt in processed food) in order to counteract hypovolemia which is possibly linked in with POTS. Getting sufficient water in is another matter. Like many, I have had a lot of probs with night sweats, increased urination when over do it and a lack of thirst too, so its difficult to rehydrate. An OT suggested smoothies to get the liquid in which was helpful. Re feeling sicker. Perhaps hangovers with its its headache, light sensitivity, body aches perhaps a good example of dehydration (among other things). Our cells simply cant work properly without sufficient water. Drinking enough didnt cure POTS but it did help with the faintness/gormless feeling. Also helped reduce sore throats and intermittent light sensitivity. But as with everything there's great variability and dehydration often catches me unawares. Of course you might be struggling with something entirely different! And apart from anything else, dysautonomia may well play a role. If you think it is, in part, dehydration, probably best to work with a doc on this so can check your electrolytes in your full blood count before hand and keep an eye on your blood pressure. As an aside I've noticed that my gums are particularly pale when I'm dehydrated - a very pale/white pink instead of a mid range healthy pink. Not sure that drs use gum colour to check hydration levels but vets certainly do.