I think that we as humans think that if we wouldn't like it, neither would the cat. I found this to be absolutely false both when bringing the kittens home and when we moved to a new house. If I left them in a smaller room, they were much more calm. It was a new territory to explore and sniff and get comfortable with, but they started out small, with just the one room. I would let them out when I was home, but it took days before they'd venture out of their safe room on their own, and then,timidly. When we moved from our apartment to our house, it took a full week before Max came downstairs. For my boys, giving them free reign of the entire living space was intimidating for them. So I don't think you should feel bad for leaving the kitten alone in a room while you're at work or at night; it's their safe place where they'll feel the most comfortable until they're ready to explore the entire house/apartment.