One nasty habit that I never dropped as I grew up has been biting my nails.
Now generally nail-biting is a sign of stress/tension/worry. But the only times I have consistently been able to stop biting them and allow them to grow long has been while doing exams at university - which I think most uni students would agree is NOT a stress-free period.
The other times I can grow them is when I am out and about getting dirty - such as a fortnight ago when I was camping in Geraldton. They have been looking good for over a week, but today they got too long so I cut them shorter in the morning. But the spell was broken - by mid-afternoon my nails had totally disappeared :( Maybe it is boredom. Anyway, I don’t expect to have nails again until around November…
It is a nasty habit and maybe you don’t want to read about it, so I’ll stop now.
Oh, and don’t even let me start on about my fingernails ;)