Noob question - why can't I resume from standby??

Hi, I have a 20" iMac with OS X Tiger installed. It works great except for the following: if it goes into standby (with white light slowly flashing) after let's say an hour, I can wake it up by pressing a keyboard key or clicking/moving the mouse. It then springs back to life and all is well. However, if I leave it overnight, the white light is slowly flashing indicating it's in standby mode, but I click the mouse, press keys on the keyboard, move the mouse, and the white light goes off but the thing doesn't come back to life. There's nothing on the screen at all, just off. I then have to manually power down by holding the power button for a few seconds and then restarting it, and it will reboot normally and all is well..... until I leave it on overnight again.

Any suggestions? I don't have any (known) hardware issues and otherwise it behaves perfectly well. It's a 1.8 ghz if that makes any difference.

Thanks much

My MBP use to do that. Quite often I use the screen very dim. My issue was that if I left the screen dimmed all the way down to the lowest setting prior to it going black and then let it go to sleep, when it woke from sleep, it was on that lowest setting. Discovered I could simply increase the screen brightness after waking it up. Give it a try and see if that's what yours is doing.

You might need to reset the power manager:
Intel iMac: Intel-based iMac, Intel-based Mac mini: How to reset the System Management Controller
G5 iMac: iMac G5: How to Reset the SMU

Thanks for the responses. I changed the energy settings so the system doesn't go into standby but the display will still turn off. This is a workaround, but not a solution.

You still need to try my first post above. What you're describing tells me it is waking up, but the screen is dimmed. Use the function key to increase the brightness after it wakes up.

Are you using a wireless mouse and/or keyboard?