The idea that drinking lots of water (hydration) is important is not controversial. Why then include hydration as an exercise? Well, most people remain dehydrated.
The function of the body that keeps physical systems balanced (homeostasis) is capable, under chronic stress, of slipping the 'targets' to less balanced levels (allostasis). Under chronic dehydration, the 'balance point' is set 'drier'. This means that if someone chronically de-hydrated drinks water, as he or she should, more urine and light urine will start to be produced before optimal hydration. In this case, if urine output is used for a guide, drinking water will stop before optimal hydration. Also in chronic de-hydration, thirst tends to drop off or become less of a message to the person.
Use of the will, and stress lead to dehydration because they 'override' the natural self-nourishing impulses of the body. After a time of not honoring thirst, thirst is no longer felt.
To change this in general of course requires an overall relaxation. More specifically, attention to water and increased intake of water is needed. If one experiments with continuing to drink a great deal of water, it is usually experienced that urination increases greatly at first then normalizes as the 'set point' of water balanced is raised back to something more optimal. Of course the value in hydration is not just a better set-point of water balance but also greater 'flow-through' with the elimination of metabolic wastes
Salt intake may be unnecessarily curbed, making it hard for stressed adrenals to maintain good water balance. See works on the (controversial) concept of adrenal fatigue.
Sometimes the displeasure waking up at night urinate contributes to dehydration. However, it is a natural cycle for the body to produce less urine during sleep at night. Stress will interfere with this cycle. Also a pelvis anterior tilt will stretch and press the bladder, and lead to a stronger urge to urinate at lower bladder volumes. As the pelvis becomes more aligned, this cause of awakening at night will recede.
There is a common comfort practice that perhaps gives hydration a mild stigma in the general culture. That is the practice of bringing a water bottle with a nipple around with one everywhere, keeping it on one's person at all times, and taking an occasional a small sip, especially before deciding to do something, or complying with a request. In hydration, there is no reason not to drink a glass or more at a time. The sipping behavior may not amount to much total intake, and the interpersonal timing suggests it is substitute oral gratification. Sipping may help with a dry mouth (from fear and sympathetic shift), and of course comforting is a necessary part of life, but in the popular imagination, hydration is wrongly associated with regression.