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Does anyone know how to center sideways text in a table?

Example:

\begin{tabular}{|c|c|c|}
\hline
& \begin{sideways}USERNAME\end{sideways}&
\begin{sideways}PASSW\end{sideways}\\
\hline
NAME1 & x & x \\
\hline
NAME2 & x & x \\
\hline
\end{tabular}

I wish to vertically center the sideways text ...