Depends on how long you intend to keep it off. Turning a laptop on means that it has to load quite a bit of data from the disk, so it's a bit of a strain on the disk. If you just put it to sleep, it won't need to do that the next time you use it. On the other hand, if you put it to sleep, it's not completely turned off, so it still needs power, although not a lot. I recall information from Apple saying that if you need your computer again in less than 8 hours, you should put it to sleep insted of shutting it down, but that was a few years ago. There are tons of discussions on this subject: http://www.dssw.co.uk/sleepcentre/threads/sleep_vs_shut_down_which.html http://forums.macosxhints.com/showthread.php?t=37771 http://www.google.de/search?q=sleep+vs.+shut+down One point is quite certain, though: you shouldn't just keep it running without turning it off or putting it to sleep if you don't use it and aren't running anything like SETI on it.

Its better to turn a laptop off when its not in use for a few reasons- First of all the flat panel illumination has a limited life span. The longer you leave it up and running, the more hours you are putting on that screen. Even though you dont have to worry about the screen going out next week, over the course of the next couple of years, leaving it on without putting the screen to sleep will cause its life span to end before the life of the PC itself. Secondly, laptops are harder to cool off with the few fans that are inside. Even though you will not experiance a meltdown through normal use, having the machine up and running while it isnt being used is unnecessarily creating heat on the components. Third, the smaller components in a laptop are not as durable as those in a desktop when it comes to endurance. Even though a laptop is more rugged and more able to withstand vibration, shock, and other enviromental hazards, the hard drive tends to have a shorter life span just through normal use. All in all using your laptop for extended periods of time is nothing to be concerned about. But when the laptop begins to die through normal wear and tear it's better to know that you got your full usage out of it vs. it wearing out just because you left it sitting on your desk running.

Machines and electronics suffer high levels of stress during start. Electronics go from cold to hot and expand. I would guess that sleep mode would be from warm to hot. I think sleep mode save me more energy overall. I put my laptop to sleep frequently because it is ready by the time I raise the lid (screen), but I would leave it running if sleep was not a option. Then startup would take time and not practical. Wasting energy and parts. I could think of one situation to shutdown. If you know a electrical storm is coming and you want to unplug. But then laptops can sleep on battery power for a long time.