"Why is it that I can gain 5lbs overnight?"

"Why is it that I can gain 5lbs overnight? HOW???”  

If we take in more calories than we can use at the moment, our body will store those calories in a few different ways. Our body will first store extra glucose in our liver and muscles in the form of glycogen. Once these stores are full, additional glucose is stored as fat. You know how athletes like to “carb load” the night before a big race? This is done to fill up their glycogen stores to prepare for the race because glycogen is easier for the body to use for energy than fat stores. The problem is that glycogen works like a sponge, holding up to THREE TIMES its weight in water, and water is heavy.

So here's what actually happens, Let's say one day all I eat a HUGE basket of french fries that has 3750 calories. I only burn off 2000 calories that day, which results in a calorie surplus of 1750 calories, approximately half a pound (one pound = 3500 calories). Logically I would think “I will see half a pound tomorrow morning on the scale” but what I will see is more like a two pound gain on the scale because of the glycogen/water response. 

French fries also have a ton of salt, which makes us retain water. This is because it disrupts the balance of sodium and potassium in the body, which affects our kidney's ability to filter extra water out of our blood. This will make us gain water weight and increase our blood pressure. So adding carbs + sodium can be a perfect storm for an immediate overnight weight gain (Remember we are talking weight **not fat)

So in our example, I’ve gained about 2 pounds from glycogen and water, and 3 pounds from water diluting the salt in my blood. That's 5 extra pounds in one day from eating something that was only the equivalent of half a pound of surplus calories. Also if you a hard workout the day before which has caused some muscle fiber tears (natural healing process) you will have inflammation, inflammation causes some micro swelling that can add even more! 
FINALLY the mystery is solved!! 

To sum up: The scale often lies! Don’t let the number on the scale that only measures your relationship with gravity have power over your emotions.