Weight can fluctuate for a variety of reasons. If you weigh yourself before bed and the following morning, the two numbers are often different, by even as much as 5 lbs. While it takes 3,500 calories to gain 1 lb., you may still see a fluctuation of weight even if you don't ingest anything during the course of an hour.
Weight gain occurs when you consume more calories than you've expended. One pound is equal to 3,500 calories. While your scale may read a 5-lb. difference in just one night's time, it's highly unlikely that you've actually gained weight, unless you happened to consume 17,500 calories overnight.
One Hour Weight Gain
It is possible for your weight to change in the course of an hour even if you haven't consumed anything. While drinking two glasses of water is equal to 1 lb. of weight, not drinking water when you're thirsty can lead to water retention and temporary weight gain as well. While it is not a permanent weight gain, it is possible for the number on the scale to change within an hour.
Other Weight Fluctuations
Temporary weight gain can be caused by water retention, menstrual periods and overconsumption of salty foods. The food you eat also weighs something. So while you wouldn't stand on the scale holding a 6-inch sub in your hand, you probably wouldn't want to weigh yourself right after consuming it either. Weighing yourself after exercise can show a lower number due to sweating and dehydration.
When to Weigh Yourself
If your weight is something you would like to monitor, you should weigh yourself consistently at the same time and under the same circumstances. Due to daily weight fluctuations, the Bally Fitness website recommends weighing yourself once a week on the same day. Weighing yourself in the morning is best, before eating or drinking anything, and since clothes have varying weights, weigh yourself in the buff.