No, eating watermelon at night will not cause any problems. Watermelon is a wonderful summer fruit and it contains 91% water, fiber and natural sugars. Because it is high in sugars, it will lead to weight gain as at night you will not be exercising the body to use up the excess sugars. These get converted to fats and will be stored-in the body. The fruit is high in fiber and will work as an excellent detoxifying agent as it helps in removing unwanted wastes in the form of stools the next day. If you are not eating anything else for dinner, then enjoy a small piece of the fruit.