Sleep & Monitor Problem

But if I turn my monitor off, then the rest of the system will either wake up or will not sleep at all
Madwen wrote on :

If I set my energy saver to sleep my system or select "sleep" before going to bed at night, it puts everything to sleep the way it should, including parking all of my 2 FW and 2 SCSI drives (though I rarely have them all on). However, in sleep mode, my monitor (Mitsubishi Diamond Pro 900u) makes this barely audible but high-pitched squealing noise that does not sound healthy. I have no other problems with my monitor.
As a consequence, I prefer to turn my monitor off at night and sleep the rest of the system.

But if I turn my monitor off, then the rest of the system will either wake up or will not sleep at all (even if the energy saver is set to make it sleep in five minutes or whatever). The only way I can sleep the system is to power off the monitor and then use the power button on the computer to make it sleep. I guess I don't understand why the system will not sleep after the monitor is powered off.

Any ideas on how I can get my system to sleep after powering off the monitor? Quicksilver with OS 10.3.1.

Madeleine