As I've mentioned before tracking weight as a benchmark of progress has the potential to be misleading. You can be doing all the right things and positively changing body shape without registering too much of a dent on the scales. Weight represents earth’s gravitational pull on the body but it doesn’t discriminate between muscle and fat.
To add further confusion the human body is approximately 60% water. This means small swings in fluid balance will have a noticeable impact on scale weight irrespective of whether you have lost fat or gained muscle.
Here are common scenarios where fluctuating water levels can disrupt your measurements:
Carbohydrates - Carb stores increase water stores. This is one reason why low-carb diets appear so effective in their early stages. A reduction in carbohydrate intake leads to a reduction in carbohydrate storage which leads to a reduction in water storage and a sudden drop in scale weight. But it’s only water and will be quickly regained if and when your carb stores are topped up.
Salty Meals - High levels of sodium increase water retention. The scale spikes up but again, it’s only water and will be lost if and when sodium levels in the body are reduced.
Hormone Balance - Natural shifts and cycles in hormone levels can cause the body to retain more or less water. Again the scale weight rises and falls but it’s only water.
Dehydration - Lost 5lbs in a sauna? It's water and will quickly return once you rehydrate. Fighters and other weight-class athletes will often take advantage of dehydration to drop their weight for weigh-in before rapidly rehydrating prior to the contest.
Measuring weight is not completely meaningless but its important to remember it does not show the whole picture. Fat weight, lean weight and water weight are completely indistinguishable once you step onto a scale. If you must track weight keep this knowledge in mind and don't be discouraged by day-to-day fluctuations which are more than likely caused by a shift in fluid balance. Instead focus on trends over time to gauge progress.