The only way to cure dehydration is to drink some more. Drinking the night before will have absolutely no effect on hydration the next day. Drink a few To determine how much water you should drink after you spin, you should weight yourself before and after you exercise to get a sense of how much fluid you typically lose. Try to drink about 1.5 pints for every pound you lose.
First, while water is good, a drink with a little sodium, potassium, chloride and other elements is absorbed faster by the intestine than pure water. This puts more water into the blood faster. You could try to have a sports drink after you exercise. Sports drinks can be helpful to athletes who are exercising at a high intensity for at least an hour. It's necessary to replace losses of sodium, potassium and other electrolytes during exercise.
You may also be suffering from a condition called hyponatremia which is a low blood sodium concentration that results during longer events when athletes drink excessive amounts of plain water. A sports drink will help cure this.
Also, there is cellular damage after any kind of exercise. As a result, there is an increased breakdown of protein during exercise. This damage is structural, and the body's structural building materials are proteins. Most disrupted cellular processes begin repair as soon as the stressor is removed. Protein consumed during exercise may help speed recovery following exercise. Plus, during extended exercise, protein can provide up to 20 percent of the muscle's energy needs because the muscle is becoming depleted of glycogen.
Therefore, adding protein to a sports drink minimizes muscle protein breakdown that would then speed muscle recovery after exercise. Thus, a little protein during and after work is a good idea.
Dr. Ron Maughan conducted many studies on hydration for athletes. You might want to try to look up some of his findings.