Should I turn off my computer or put it to sleep?
Answer: This question can invoke quite a bit of debate, but my short answer is to put the computer to sleep each night or any time you leave the computer for more than 15 minutes. If you are not going to use the computer for more than a day (such as leaving it for the weekend), it is best to turn the computer off.
Idle 15 minutes to 24 hours: Sleep Idle 24 hours or more: Shut Down
When a computer is in "sleep" or "standby" mode, it consumes almost no energy, so the difference between letting it sleep overnight or turning it off during the night is negligible. However, if you have a laptop running off a battery charge, the difference may be noticeable. I have noticed that my laptop's battery life only lasts about five to six days in sleep mode, but will last for weeks if the computer is off. Therefore, if you are not going to be using your laptop for more than a few hours and do not have AC power available, it is best to turn the computer off.
The most important way to save energy is to not leave the computer running normally when you are not using it. I have my computer set to automatically sleep if it is idle for 15 minutes and my screen dims after 5 minutes. This way, I know that I won't accidentally leave my computer running for a long period of time. I also put my computer to sleep manually if I know I am going to be taking a break for more than 15 minutes.
Finally, if you put your computer to sleep every night and hardly ever turn if off, it may be good to at least restart the computer every few weeks. This may help improve performance in case some errant processes are consuming unnecessary RAM or CPU time.
Windows users: Read more about the differences between Standby and Hibernate modes.
Mac users: The keyboard shortcut for putting your computer to sleep is Command-Option-Eject.