As of 100.14.19 nVidia driver (nvidia-glx-new) has been introduced an experimental support for
ondemand vblank interrupts.

The X xorg.conf subsection for the video driver needs this line:
Option “OnDemandVBlankInterrupts” “true”

I just tested it and must say it is indeed effective! the 60 wakeups per second that wakened up my system before due to the vertical refresh (of 60Hz) are now disappeared! I still have to find out how mouch does this mean in terms of watts/minutes saving but it is indeed a good news!

nvidia-readme snippet:
Normally, VBlank interrupts are generated on every vertical refresh of
every display device connected to the GPU(s) installed in a given system.
This experimental option enables on-demand VBlank control, allowing the
driver to enable VBlank interrupt generation only when it is required.
This can help conserve power.

Default: off (on-demand VBlank control is disabled)