After being on all day, my imac will not restart or shut down.

After being on all day, my imac will not restart or shut down. I have to push the on/off button wait for it to reboot then shut down. If I shut down or restart before a eight (8) hour period it will shutdown and restart. I have tried the SAM & PRAM, No Avail! Any Insight?

THANK YOU VERY MUCH,

Steve