Question: Is it best to turn off your computer at night, or
let it run all the time? 

Answer: You may be a person that turns your computer off for the night. But is it wise to do so? There are two sides to this story.

Turning a computer on and off changes the temperature of its components, which stresses them. The fan keeps the temperature constant while the machine is running. Except for
the monitor, which you should turn off or put in power-saving mode when not using it for an hour or more, computers don't use much electricity when run constantly.

On the other hand leaving a computer on can wear down the always-spinning hard drive, unless you have one that goes into a sleep mode when not being used for a period of time.
Dust on the computer’s fan can make it an inefficient cooler.   If you leave the computer on you will also need to reboot periodically to flush the memory of  digital “stuff” that programs have left behind when you close them. 

I don’t recommend leaving a computer turned on unless you are using a UPS (uninterrupted power supply) surge protector.  Leaving a computer unsupervised is not a wise decision because it only takes a real good series of surges during an on and off  power surge session to damage parts of the computer.  If you are using a UPS to support your system then it will protect it from “dirty current” and from small on and off surges.

I leave my computer on all the time supported by a UPS and a sleep mode.  The exceptions are if I plan to be away for a day or so or there is an approaching electrical storm. 

Question:  What about using screen savers to protect my computer monitor. 

Answer:  I say hogwash to screen savers.  Screen savers are an excellent feature for businesses to use to help sell their computers.  For the individual, the screen saver is a memory-gobbling piece of junk.  If you are having trouble with program crashes you might consider not using screen savers except for special occasions.   Screen savers can be very interesting and beautiful but they have some faults too.  Will they stop screen burn?  Well probably so, but the best way to protect your screen is to turn off the monitor at the surge bar when you plan to be away for a while.

The monitor needs to be turned off any time you are away from the computer for a significant period of time.  It is important however to plug the monitor into a separate surge bar so it can be turned on an off independent of all other equipment.  Do not constantly use the monitor switch or you may soon be looking an expensive replacement.