Can I say,<br/><br/>(a) He was studying until late night. /at late night.<br/><br/>(b) He was studying late at night.<br/><br/>(c) He was studying at his room. It was late.<br/><br/>(d) He was studying late although it was late night. <br/><br/>(e) He was studying until (very) late. <br/><br/>(f) It was a very late night.