What could be causing me to lose weight overnight?

I am 20 years old, i weigh 150 and i'm 6'2". Every night i lose weight. Some nights i lose more weight than normal. I can tell by how my mid region looks before bed. I've made it a point to look at myself so i'll know its not just all in my head. Some mornings i'll wake up and i can see my ribs and sometimes its kinda sunk in on the bottom rib. I weigh 150 and im 6'2". I eat healthy and i eat every three hours of the day as well as snacks throughout. The only thing i can contribute it to is loss of water weight and quality of sleep. I don't drink that much water throughout the day i've just heard that alot of times weight loss overnight is water weight. I don't know if quality of sleep could make me lose weight but when i sleep better i don't lose as much.