We consider negabent Boolean functions that have Trace representation. We completely characterize quadratic negabent monomial functions. We show the relation between negabent functions and bent functions via a quadratic function. Using this characterization, we give infinite classes of bent-negabent Boolean functions over the finite field $\F_{2^n}$, with the maximum possible degree, $n \over 2$. These are the first ever constructions of negabent functions with trace representation that have optimal degree.