Rendering Over night? Will it sleep or not?

Would it harm the machine to keep going that long?

I routinely run encoding or rendering overnight with no ill effects.
It should not be a problem to run a computer for weeks/months on end.....

Hi-

The rendering process will keep the machine from going to sleep- this should not be a problem.

Will the rendering continue till it's done and then will the computer sleep an hour later?

That is how it should be expected to work.
I have a machine that I use for rendering, and it sleeps 30 min after rendering is done.

I tried this and when I came back the next day the fan was going strong. The rendering seemed to work but actually only went about three quarters of the way.

This indicates that the machine encountered a problem.

And why was the fan going if the computer, supposedly was asleep?

It was frozen? It was not normal sleep....
How did you resume?
Did it require a hard restart?

I had to shut it down and restart. Couldn't get it to wake up.

It could be that the system started it's automatic maintenance routines around 3:00-5:00 AM.
If the maintenance routines haven't been run in a while, the process could have cause the freeze (really a system crash), by adding extra CPU demand during an already demanding process.

I would check your hard drive with Disk Utility (Verify/Repair disk). This is simply because a crash can leave directory files in a mess. You will need to boot to the OS X install disk to use the "Repair Disk" function.
Using Disk Warrior, if you have it, is better.

You should be able to run a process overnight, but just to be sure maintenance tasks don't interfere, use Terminal to run your maintenance scripts manually.

Open Terminal.
At the command line prompt type

sudo periodic daily weekly monthly and press Return.

Enter your admin password, and press Return.

The processes will be running, and will be done when the command line prompt reappears.

You can then load your project, and let it go.
It should all run without a problem.

FYI- more explanation on the maintenance scripts:
http://www.thexlab.com/faqs/maintscripts.html

I would probably just change the sleep setting to "Never" for the night you leave it going just to be safe. You can then always change the setting back to where it was once the rendering is done.