When stepping away from your computer, there are 3 options available. You can shut down, hibernate or sleep the computer. The speed and power requirements vary for each options. Sleep uses more power, but less time to resume. Hibernate uses less power but resume slowly. Most computers seems to resume from hibernate faster than a complete boot after shutdown.
Shut down closes all the open programs and puts the computer to a power-off state. When you turn on your computer after shut down, it’ll need to initialize hardware and other startup programs. People generally shut down computer every time, but it is not necessary, you can use hibernate instead. However it is recommended to restart windows occasionally.
Hibernate saves the current state and power-off the computer. That means you can resume from wherever you left. When you hibernate, all the data in your RAM is dumped to your hard drive. When you resume, data is loaded back to RAM from hard drive. Personally I prefer to hibernate my computer, when i step away for long time. But some programs fail to work after resuming from hibernate state. In such cases you should go with shut down.
Sleep puts your computer to a low power state. It keeps your RAM working, and turns off other hardware. Thus it needs more power than hibernate. However, you can resume from sleep mode within seconds. So prefer sleep mode When you are leaving for short break, like leaving for a coffee.
In conclusion, use sleep when going for a short break. Otherwise use hibernate unless some programs fail after resuming from hibernate, in such case use shutdown instead. Also shutdown or restart your computer occasionally.