I have my iG5/10.3.9 set go to sleep when I hit the power button on the back. To wake it, I have to click a mouse button- moving the mouse doesn't do it.
And that's the way I like it.
Suddenly, over the past 24 hours, when I sent it to sleep it would start to sleep, then wake up. A second sleep (still using the power button) worked okay. But moving the mouse would wake the Mac straight away. Nothing else had changed. Power saving prefs were unchanged.
After a small amount of faffing I remembered the Realities of Life; restart. And it works fine again.
Isn't it nice how some tricks Just Work, every time?