Why do I bite my nails and grind my teeth?

Body-focused repetitive behaviours, usually shortened to BFRBs, are the umbrella term for self-grooming actions that end up doing some damage. The Cleveland Clinic's list of types includes onychophagia, which is nail biting, and bruxism, which is teeth grinding. So the pairing you've noticed is already built into how these behaviours get classified.

The TLC Foundation for BFRBs is more specific. On its nail-biting page it notes that alongside hair pulling and skin picking, frequently co-occurring behaviours include bruxism and cheek biting. Having both is common enough to be written down as an expected feature rather than a coincidence.

The most immediate thing they share is the job they do. Both take a charged nervous system and give it something to do with the charge. The Cleveland Clinic describes BFRBs as possibly serving emotional regulation, a way of handling or avoiding uncomfortable feelings like discomfort or boredom, and also as a distraction when you're either under-stimulated or over-stimulated.

There's shared ground further down as well. The same source points to brain structure and genetics, specifically the parts involved in reward processing and emotion, and to the chemical regulation of serotonin and dopamine. That's the honest level of detail available: contributing factors rather than a single cause, and none of it settled.

The two also behave alike in the moment. Both can run without you noticing, which is why people describe them as automatic, and both deliver a small hit of relief that teaches the loop to return.

This is worth being clear about, because it's a natural assumption. There's no evidence that biting your nails makes you grind your teeth, or the reverse. What the sources describe is co-occurrence, meaning the two turn up together more often than chance would predict, without either one driving the other.

There is one real-world overlap worth knowing about. The TLC Foundation notes that nail biting can contribute to dental problems and to temporomandibular dysfunction, which means trouble at the jaw joint. Mayo Clinic separately lists awake-time mouth habits, including lip and cheek biting, among the risk factors for awake bruxism. So a habit that keeps your mouth busy may add to the load on a jaw that's already working hard, which is a shared burden rather than a cause.

The useful implication is that you probably don't need two unrelated projects. If both habits ride on the same arousal, anything that lowers that baseline takes pressure off each of them. Steadier sleep and genuine pauses in the day are unglamorous, and they tend to do more than tackling each habit in isolation.

The habit-specific parts still matter, because the two need different things in the moment. Hands respond to having another job. A jaw responds to letting the teeth come apart and breathing out slowly. Many people find it easier to work on the more bothersome one first rather than both at once, since progress on one often takes some heat out of the other.

Both have their own physical consequences worth protecting against. A dentist is the right person for worn teeth or a sore jaw, whichever habit is doing the damage, and it's reasonable to mention both at the same appointment.