I've disagreed with this ^^ for years, even though many people preached it. I still stand against it and advise all of my friends and customers to SHUT DOWN at the end of the day. Take for example my home PC - I don't get home from work until at least 4pm every day; why in the world should it be running, wasting power and wear-and-tear on my components, while no one is there to use it? I also go to bed each night around 12 and don't have time to use it in the mornings (if I want to check something fast I can use my iPad anyway). They way I see it, my computer is on for at most 8 hours a day this way, rather than 24. Same applies to my work PC, except in reverse.
And it appears I'm not alone...
http://www.pcworld.com/article/240118/turn_a_pc_off_or_leave_it_on.html
My laptop is another story, though - I usually leave my MBP "on." I just close the lid and put it to sleep whenever I'm finished.
You're not alone. it is a myth that "starting your computer is stressful on it".
this myth dates back to old AT days. when computers had mechanical power switches that actually activated the power supply directly by closing a circuit. A lot of failures in computers early days happened when turning on the computer because well, it was early tech, and mechanically throwing a switch which suddenly jolted power, often unpredictably down to every device all at once.
once the move to ATX motherboards, tech and power supplies came around, much of this was eliminated. Powersupplies were no longer started by physically closing the circuit but by electronic controls. Believe it or not, your power button on your computer, goes to your motherboard first, which then tells your PSU to start up.
As for "load on the system" that is caused by a computer start up? it is no different than say, running a game, or anything else that would need to access data quickly from your disc, load it to memory, and provide constant power. There is no additional wear to components by starting up windows / OSx than there is starting up World of Warcraft or your favourite shooter.
Quite the opposite is true by leaving your computer on constantly however. When you turn off your computer, it does nothing. there is little to no power usage (modern computers always draw current unless unplugged). What ends up happening, while your computer is running is well, all the parts in use, continue to be in use. Your power supply continues to be under load. your video card, CPU, and all your fancy expensive components continue to be utilized. Heat is still generated, fans continue to spin. In short, leaving your computer ON, continues to use the limited lifespan of all parts. While todays hardware's lifespan can be measured likely in Decades, this wasn't always the case. Even hard drives today are a primary point of failure because constant non stop use wears down mechanical parts. Heat from active parts always has a negative, even if minuscule affect on all computer parts. Plus not to mention the sheer amount of power draw and waste.
And your computer you leave on during the day isn't just sitting idle as well. Many modern Operating systems perform "idle tasks". windows for example when detecting long periods of inactivity can and often does hard drive maintenance such as defraging
SO, is it safer to turn your computer off or leave it on between uses? Turning it off should prolong the life of any computer if you have no need for it to be left on.
Newer computers with fewer and fewer moving parts, the Macbook Air for example only has a single moving fan that more often than not is powered down, really won't matter too much in the long run, aside from the obvious power consumption.
Going to sleep is pretty close to powering down in regards to power consumption and load on the power usage.
What shutting down provides is clearing out the RAM. With no power, RAM is empty, So when you start your computer up the next time, the computer will go through it's boot sequence and load all fresh new data from the hard drive to system memory. This is why when you call for Tech support, they often ask "have you restarted / shut down your computer". sometimes after too long running, without realizing it, programs leave bits of data behind. or lock little bits of ram accidentally (memory leaks) and overall performance might degrade or conflicts might arise. Rebooting and shutting down will always clear the contents of RAM.