。
I'm a night owl.
Literal
I [topic-は] night-type [is-な]-[explanation-んだ].
夜型 ('night-type person, night owl') is a compound combining 夜 ('night') + 型 ('type'). The opposite is 朝型 ('morning type, early riser'). なんだ combines the attributive copula な (for nouns and な-adjectives) + んだ (explanatory).