For the majority of people by not going outside t would be unhealthy. However, there are always exceptions. Some people who worked the night shift actually manage to be nocturnal -- but really, it's their daily routine so not sure if I would consider it "nocturnal".
One reason why it is unhealthy includes, but not limited to is depression. The lack of sunlight during the day is linked to depression. Our brain naturally produces melatonin (a sleep hormones) that can contrast the difference between day and night. If you're only expose to the dark then there will be less production of melatonin. More brain chemicals like serotonin would decrease, causing you to be easily irritated and moody. Furthermore, the lack of sunlight may deteriorate an individual's mental health over time. Vitamin D is super important. I know many introverts are vampires but gotta say hi to the sun from time to time joy)
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From @phoenixgold