How often do you leave your TV on overnight? Or your clothes dryer? It makes only slightly more sense to leave your computer on when you're not using it.
Once you set up your sleep mode, you've made the most significant dent. But off mode still uses less energy, and those small savings can add up.
As a general rule of thumb, shut off your monitor if you plan to be away from your computer for at least 20 minutes; and shut of the CPU if you know you'll be away for 2 hours [source: Energy Savers]. As an added bonus, shutting down will stop your computer from generating heat, saving both your cooling components and some kWh on your air conditioning bill.
Of course, there are a few circumstances in which shutting down is not a great option, like when you need to be able to access your machine remotely or you're in the middle of a big download. However, even these complications have some solutions that allow you to take full advantage of your computer's power-saving features. For example, the BitTorrent client uTorrent can turn your computer off when a download completes; and an application called Wake-On-LAN can let you power-on your computer remotely [source: Pash].
And while you're getting in the habit of shutting down, go ahead and plug all of your components into a power strip that you can unplug overnight to eliminate that pesky "phantom" power draw.
Up next, if you already practice green computing and you're still looking to save energy, it may be time to upgrade.