The real reason I let my hair air-dry before laying my cranium on a silk pillowcase (I gave in) is that I would rather not wake up with a bald spot surrounded by a wave crest of a cowlick. Dealing with this tumbleweed when it's not damp is hard enough. But we've all heard the other, non-superficial arguments: headaches. Breathing in mold spores. Etc.
Here, we bring together science, beauty, and quite literally your mom, and put them in dialogue with one another.
My Mom (and Probably Your Mom Too)
I am very hesitant to call this an old wives' tale, because my mom wouldn't be too thrilled with that description. For as long as she has stopped being personally responsible for my hygiene, my mom has tsk-tsked my habit of sleeping on towel-squeezed hair as "uncomfortable" and "you'll regret it someday when your head hurts all the time." Relax, Mom.
The Stylists, Who Get to Talk Second Because It's What's Outside That Counts
In contrast to concerned parents everywhere, the professional hair people are like, "Yeah, sure. This is fine." Benjamin Mohapi, owner of the Benjamin Salon, says "going to bed with wet hair can be great for some textures," but advises against it for those with more mass/volume, which can result in waking with still-wet sections and a weighed-down feel. The key, then, to avoiding a tragic bump is being deliberate about it. Anh Co Tran, L'Oréal Professionnel International Hairstylist and co-owner of Ramirez Tran Salon, suggests pressing excess moisture from the hair and sleeping on a loose braid. In the morning, touch up pieces with your preferred hot tool, concentrating on the high-visibility areas surrounding the face and crown.
The Nerds in the Back, AKA Science
A fact, not an accusation: Your pillow is probably nasty with oil and dead skin cells, because you're not changing it as often as you should. (Half the room goes "Same," while looking sheepish.) And when you introduce something damp, you're creating an environment ideal for the growth of mold. The headache risk, on the other hand, comes with less solid of a scientific backing—there isn't enough evidence to support the hypothesis that wet hair makes it more difficult to regulate body temperature, thus causing pain. But studies do suggest that skipping the blow-dryer can trigger colds, though the virus would have to be already present.
Conclusion, or What You Came For
If you can be responsible about laundering your pillowcases and arranging your hair in a way that won't make it come out like Mugatu's but only the left half, then by all means—do it. Prove your mom wrong. But maybe buy some stock in Tylenol just in case, 45 years later, you're like "Ow, what is this hammering in my head?" all the time.