If you're trying to lose weight and have the habit of weighing yourself frequently, you'd know just how fickle these bathroom scales can be. Your weight can fluctuate a few pounds overnight! Now, it's completely normal for a person's weight to vary throughout the day, but gaining four or five pounds just like that? What gives?

The truth is there are many reasons why the numbers can fluctuate so much. Here are eight of the most common ones -- the next time the scale doesn't cooperate, you'll know why:

1. You're wearing heavier clothes

On average, clothes add up to 2 pounds to your actual weight. While summer clothing is relatively lightweight, the heavier jeans and jackets could add more than 2 pounds. For a more accurate reading, remember to take off your clothing and shoes.

2. You just finished exercising

After a high-intensity workout like running or a high-energy spinning class, you might see a 5-pound dip on the scale. The drop isn't a true weight loss; it's a loss of water weight -- you burn glycogen calories (carbs), which hold water weight -- and your weight will go back on once you rehydrate. You need to drink 16 ounces of water for every pound down after a workout to replenish your body's fluids.

3. You consumed too much sodium

Eating sodium-laden foods like canned soup, frozen meals as well as condiments such as soy sauce and MSG can lead to temporary water-weight gain. Your body retains additional fluid to flush out the excess sodium, and this explains why your scale weight creeps up the day after a salty meal.

4. You're filled up with fiber

Eating fiber-rich foods like vegetables, fruits, whole grains, beans and nuts can make the scale numbers go up, but don't fret -- it's only temporary. Fiber is not digested or absorbed by the body. Instead, it provides bulk in the diet and prevents constipation, and once it's done its job, you should see the scale go down again.

5. You're experiencing PMS bloating

If you put on a couple of extra pounds right before you start your period, PMS is likely the culprit. Researchers think it's caused by hormones: A shift in estrogen and progesterone leading to water retention and swelling. The good news is that it's not permanent. After those few days, your weight will return to normal.

6. You're constipated

Full bowels can add up to 2 pounds of extra weight, so the numbers on the scale may rise if you're constipated. To eliminate this problem, eat lots of fiber -- about 30 grams a day -- plus plenty of water and exercise to move things along. Once you're regular again, the added weight will fall away.

7. You went drinking last night

Alcohol can add a lot of sneaky calories -- and weight. A 12-ounce can of beer, for example, has about 150 calories; a 5-ounce glass of wine has approximately 125 calories; and a 1.5-ounce shot of hard alcohol has about 100 calories.

8. You're stressed

Having a stressful day can affect your appetite -- you may snack more or reach for comfort food to calm down -- and it can make your scale weight fluctuate.