Is it still necessary to shut down computers?

From a software perspective, an operating system and the programs you run on it tend to accumulate all sorts of cruft over extended periods of use - temporary files, disk caches, page files, open file descriptors, pipes, sockets, zombie processes, memory leaks, etc. etc. etc. All that stuff can slow down the computer, but it all goes away when you shut down or restart the system. So shutting down your computer every once in a while - and I do mean actually shutting down, not just hibernating or putting it to sleep - can give it a "fresh start" of sorts and make it seem nice and zippy again.

However, different computers and OS's are not all equally affected by this phenomenon. Generally, a computer with a lot of RAM can go for much longer than a computer with only a little RAM. A server, on which you just start up a few programs and then let them work, will be fine for much longer than a desktop computer, where you're constantly opening and closing different programs and doing different things with them. Plus, server operating systems are optimized for long-term use. It's also been said that Linux and Mac OS tend to run for longer than Windows systems, although in my experience that mostly depends on what programs you use on them, and not so much on any differences between the kernels of the operating systems themselves.

From a hardware perspective, hard drives, because they have moving parts, will age when they are kept powered on. Silicon chips age with heat and power on cycles. Even though the operating system will run without a problem, the hardware will age when left on and when initially powered on.

The most obvious effect of leaving a computer running rather than shutting down can be seen in the electric bill. A computer will still draw power while in sleep mode (significantly less than when running), but if it's in hibernate mode it won't draw any more power than when it's off.

Other than the energy uses mentioned, further disadvantages of running your computer continously (i.e. not on standby) when not using are:

• Fans get clogged up with more dust which can eventually cause overheating
• More risk of power cuts/surges that can potentially do damage (mainly to desktops). This might still be a problem if it is on standby though.
• Computer may be more vulnerable to remote attacks if you happen to have unpatched/susceptible software and an inadequate firewall. Of course you want to make sure that your computer does not have this problem regardless of how much you use it.

Advantages:

• Less heat up/cool down cycles that eventually damage certain components

All components like HDD, DVD-ROM's have a rated MTTF defined i.e number of spinning hours before failure, make sure you check the ratings for accurate figures. Most laptop HDD's used have thousands of spin down cycles before they die down.

As with all mechanical devices, more number of power down cycles causes more wear and tear than keeping it actually running all the time.