  By combining complementary monitoring observations spanning long, medium and
short time scales, we have constructed power spectral densities (PSDs) of six
Seyfert~1 galaxies. These PSDs span $\gtrsim$4 orders of magnitude in temporal
frequency, sampling variations on time scales ranging from tens of minutes to
over a year. In at least four cases, the PSD shows a "break," a significant
departure from a power law, typically on time scales of order a few days. This
is similar to the behavior of Galactic X-ray binaries (XRBs), lower mass
compact systems with breaks on time scales of seconds. NGC 3783 shows tentative
evidence for a doubly-broken power law, a feature that until now has only been
seen in the (much better-defined) PSDs of low-state XRBs. It is also
interesting that (when one previously-observed object is added to make a small
sample of seven), an apparently significant correlation is seen between the
break time scale $T$ and the putative black hole mass $M_{\rm BH}$, while none
is seen between break time scale and luminosity. The data are consistent with
the linear relation $ T = M_{\rm BH}/10^{6.5} \Msun$; extrapolation over 6--7
orders of magnitude is in reasonable agreement with XRBs. All of this
strengthens the case for a physical similarity between Seyfert~1s and XRBs.
