I am a lifelong insomniac, so might have a different approach to sleep patterns to yourself, OP. I am frequently awake between 10pm and 4am, although I can honestly say that I find the idea of sleeping from 6pm to 10pm a bit odd. These are the hours that generally everyone is back from work, eating together, sitting together etc. By sleeping then, I'd be asking myself - is she avoiding interacting with the rest of the household? And if so, why?
I quite like being awake in the small hours. There is a peace and serenity to that time that occurs at no other time of the day. No traffic noise, the rest of the household asleep - there is a comfort I find in those hours. Of course, how I use those hours probably differs from how she uses those hours. I catch up on my reading, plan the next day, write shopping lists - quiet activity. Sometimes I iron, or clean the kitchen. How does your daughter spend those hours? Does she find she can study in the quiet? Or not? It really does depend on how she spends those hours whether i would worry about her or not.
She may be an insomniac altering her waking hours to match her body's needs. But - she could be depressed and that is interfering with her natural sleeping pattern. Don't base your idea of her natural pattern on the past - mine has taken some fairly major shifts over the years. (I still think fondly of the years when I would sleep through the night but be wide awake and alert from 5am - them were the days
!) You need to talk to her about what is causing her current pattern.
As to pulling her weight domestically - my main concern would be that she is not disturbing anyone in the household by being active between 10pm and 4am. I limit my activity to quiet tasks so as not to disturb everyone else (hence the ironing - very quiet activity). The 6pm-10pm sleep might be an avoidance tactic. If I felt it was, I'd be waking her and handing her jobs to do. If I felt it wasn't, I'd be expecting her to pull her weight BEFORE 6pm. If she wants to be treated as an adult then she has to pull her wight like an adult.
Basically, you need to talk with her. Establish why her pattern has settled into this shape. Establish if it is a problem. Make clear she must not create problems for the rest of the household (e.g. disturbing their pattern with noise). And make clear that she cannot shirk domestic chores.