The answer is gravity along with not rinsing properly and using the wrong shampoo. First, figure out if you have oily hair, normal hair (combo) or dry hair. If it’s oily, which is probably the case, it’s a challenge to keep the just-from-the-shower bounce overnight. Your scalp is already at work producing its natural moisturizer, which can make hair look flat by morning.
Find a mild volumizing shampoo geared for your hair type. Wash your hair carefully at night, rotating the shampoo over your scalp and roots with your fingers. You want to focus on the roots more than the ends. Use warm (not hot) water to rinse, and be extra thorough around the crown and sides. If you condition, apply that next (on the ends of hair, not the roots) but make sure it’s formulated to add volume.
Don’t brush your hair right away — gently pat with a towel to take away most of the water. Comb it about 15 minutes later. If you’ve got a blow-dryer, give it a quick hit, focusing on the roots. It’s a good idea to blow dry with your head upside down for volume.
If your hair is long enough, take a scrunchy and pull it all to the top of your head and make a loose ponytail. It’s important not to put tension on the hair — don’t make it tight. You’re just gathering the hair so that when you lay down, it’s stretched out on the pillow, not trapped beneath you.
You’ll wake up with more volume, but you may have to wet hair just a little to style it for the day. Be sure to use a volumizing mousse or spray to lock in the bounce.