If youre the PC guy in the family or some other group, youve probably been asked this question: "At night should I turn my computer off?"
Ever since normal people have had computers, this question has made the rounds with a variety of "expert" answers. The early versions presented a trade-off between power consumption and stress on components. Turning the computer on or off, so the theory goes and accurately I believe, is the most physically stressful thing you do to it all day, so I think most experts have advised that its better to leave the system on.
But in the age of the Internet, and especially in the age of broadband connections, theres a new angle probably more significant in the average case than all that wear and tear stuff. Some would claim that if you have one of those "newfangled" always-on broadband connections, then youre exposing your PC to more attacks than if you were to shut it off during periods of disuse. (Am I disparaging this attitude with my tone enough? Can you tell where Im going with this?).
To sum up in advance, this idea places all the emphasis in the wrong places. If your PC is vulnerable to attack, the answer isnt to stay offline more; you should plug the hole. If its insecure, it will be insecure for the time you use it, and that will be plenty enough time for someone to compromise it.
I should mention where I stand on the power vs. stress angle. Its better for the health of your system to just leave it on. The power consumed is, at worst, comparable to leaving a light bulb on, especially with modern Energy Star components, disks that spin down, monitors that shut themselves off, and operating systems that know how to manage power, even on desktop systems. (Incidentally, light bulbs will also live longer if you leave them on, not that I advocate gratuitous waste of energy.)
There are, however, a number of other issues that should, in some circumstances, be considered. Some PCs are loud and you might not want the ambient noise. Also, PCs can generate heat, although a modern desktop PC, if not actually doing anything, should manage its own power well enough not to heat the room. (This is certainly true of Windows; I dont know how efficient power management is in other operating systems. If you want to learn more about how Windows power management works, read this article.)
Because I no longer consider myself plugged into the hardware market, I asked my longtime colleague Nick Stam, Executive Senior Technical Guy at both PC Magazine and ExtremeTech. I specifically asked him about a newer theory Id heard that components are reliable enough in stress conditions that leaving them on all the time raises the likelihood that you will encounter problems having to do with long lifetimes, i.e., that youll run into the mean time between failures. Nick has no more quantitative data than I, but he too suspects that the stress of startup and shutdown is more of a real-world risk than long life.
In recent years, the part Ive seen fail most often in PCs is the power supply, definitely the part that gets stressed the most at power-up and power-down, but also the one with the most mechanical stress on it.