Macbook Pro goes into safe sleep instead of (standard) sleep

Recently I've started connecting my 2.5GHz Intel Core 2 Duo (pre-unibody) Macbook Pro 17" to an external monitor (via DVI).

Since then my computer goes into Safe Sleep instead of Sleep (it's plugged into power).

This is an annoyance I'd rather avoid because even if all programs relaunch:

• I have to wait for them to launch (longer)
• Programs like Excel seem to have been quit in an unorthodox fashion: when it starts back up I get an error notice that it was shutdown inappropriately & the doc which reopens is a rescued version of the doc I had open before the safe sleep.

Any clues anyone? Thanks!