I'm not 100% sure I know what I'm talking about in this question, so
please bear with me.

I have a Studio XPS 8100 desktop running Win7 Pro x64. I recently
started to let my computer go to "hybrid" sleep" at night instead of
powering off as I have always done.

As I understand it, hybrid sleep means that the computer initially goes
to sleep (S3, aka standby)and then, after a user-selected period of
time, the computer goes to hibernation (S4).

I have noticed that when my computer goes to sleep (S3), just before the
hard drives power off, the fan on my video card revs up for about 4
seconds. The fan also revs up when my computer comes out of sleep. (So
far, my computer has not 'slept' long enough to go into hibernation.)

Is it normal for the video card fan to rev up like this? Why does this
happen?

Daddy