To add to motosubatsu's answer (whose main premise - it depends on "why" - I agree with) - it could also be a couple of other situations:
They are, indeed, as the question mentions, stressed. But it's not work that stresses them - it's Real Life. They use work as an escape, and/or as a stress relief valve. It could be an unresolved issue in their daily life, or it could be grief they are trying to drown out.
There's no good resolution to offer here, although - if you have a good rapport with them personally, outside of just managerial role - you may want to ask if that's the situation, and suggest counseling or whatever other services your company offers. That is not guaranteed to be recieved well as such people are often in denial about the situation.
The "my work is enjoyable" person.
Some people just genuinely enjoy doing what they do for a living. This is quite common among software developers; who code in spare time as a hobby.
If the work (especially coding) they do at work is something they flat out enjoy, as a challenge/craft, they may very well be intrinsically motivated to work extra, just for sheer exhilaration of it. I've pulled all-nighters before, and in all honesty only part of the reason was deadlines and show-offing. Part of it was, I truly, genuinely didn't want to stop working on a problem that was taunting me and teasing me.
There's really no good solution here, short of killing their drive by giving them boring &^&^ to do. Not a good managerial approach, if you ask me. A person like that is also ideal candidate for remote/flex-time work - they are intrinsically motivated already.
The cultural pressure
This is usually combined with some of the situations in motosubatsu's or my own answer; but is reinforced by the fact that the person was shaped by a culture of extra-hard work. Perhaps they grew up in Japan (which is reputed to heavily have such a culture). Perhaps just overly-conscientious parents raised them that way. Perhaps all their friends do this overworking because they are in start-ups or financial companies which both encourage it, rightly or wrongly; and they absorb these ideas from their life peers.
The approach here is to explain that your company culture differs even if this isn't what the person is used to.