You almost always weigh more at night. All the food you eat and drink (not to mention any sodium eaten during the day). It's best to weigh in the same scale, same time, same day and same clothes for the most accurate reading. Your body is made up of 60% water, so it can fluctuate hourly (and that is normal). Things such as medication, food, water, exercise, sodium, stress, etc can all effect your weight. Don't worry about the night time weight, it's normal.