13. Simplify $\log _{\sqrt{2}} \sin \frac{7 \pi}{8}+\log _{\sqrt{2}} \sin \frac{3 \pi}{8}$, the result is $\qquad$ .
13. -3 .
$$
\begin{array}{l}
\text { Original expression }=\log _{\sqrt{2}}\left(\sin \frac{\pi}{8} \cdot \cos \frac{\pi}{8}\right)=\log _{\sqrt{2}}\left(\frac{1}{2} \sin \frac{\pi}{4}\right) \\
=\log _{\sqrt{2}} \frac{\sqrt{2}}{4}=\log _{2} \frac{1}{2} 2^{-\frac{3}{2}}=-3 .
\end{array}
$$