As the owner of two dogs I've often witnessed their
episodes of REM sleep when they launch into
re-enactments of the day's pursuits and appear to
be chasing something. The dog in this video is an
extreme case. His legs move much faster than the
usual twitching that I have seen, so that he appears
to be engaging in a full-tilted "gallop", only lying
down. Suddenly, possibly frustrated because he is
not catching up to whatever he is chasing, he does
a kind of "sleep run", which is unfortunately shortlived
because there is a wall in the way. On colliding
with the wall, he wakes up and does that "I am SO
embarrassed" thing that dogs seem to have acquired
from co-habiting with humans with cameras