It's really very easy. Follow the simple instructions given below:
1. Go to Apple Menu > System Preferences > Energy Saver
2. Set the sleep timings to low (we recommend 2 mins for display, 10 mins for sleep)
3. Check the boxes next to 'put the hard disks to sleep when possible' and the display brightness options.
4. If using a laptop, make sure you adjust settings for both power and battery states.
There are several handy apps like Caffeine that allow you to override these settings with one simple click - perfect for that important presentation when you really don’t want your laptop going to sleep!
Microsoft offer some handy instructions for Windows 7, Vista, and Windows XP users.
Peripherals and the WiFi sensor also drain energy from your computer, so disconnect or turn them off when not in use.