I drink water throughout the day, but since you usually don't drink during night while sleeping, does that start to dehydrate the body?

NO!!: If you have normal kidneys they quite tightly regulate the amount of fluid and the salts retained in your bloodstream. When you stress your body physiologically over a somewhat prolonged time period - I.e. Run a marathon, participate in an Ironman competition, play sports very vigorously- you can become dehydrated if you don't replenish your fluid and electrolytes frequently.