I’m as guilty as anyone, but I can no longer plead ignorance: leaving your computer monitor on overnight can waste a lot of electricity, even if it’s a power-saving "Energy Star" model.
I used to be in the habit of keeping my monitor in "sleep" mode overnight, thinking that I was using only the barest trickle of electricity. But it turns out that it’s more than a trickle: to earn an "Energy Star" label from the U.S. EPA, a computer monitor can use as much as 15 watts of energy in "light sleep" mode, and as much as 8 watts in "deep sleep," which kicks in after an extended period of inactivity.
An 8 watt drain really adds up—over the course of a year, it’s like leaving a 100-watt bulb on for a month straight. So if you want to be a real energy star, you ought to turn off that monitor.