May be it is practical to always be available, but it can be irritating for you to be constantly notified of new emails, messages or status updates. Of course, you can switch off your phone or set it to flight mode, but that would render you completely inaccessible. Even switching to silent mode or with only vibration active does solve the problem to an extent, but just for you and not for those trying to reach you.
But iOS 6's “Do not disturb” function gives you a way out. It helps you to determine whom you want to be accessible to; you'll find it in the "Settings" menu. An icon stating the function is active will now be displayed in the status bar with a moon symbol. However, its configuration menu is somewhat hidden. Go to "Messages | Do not disturb" and activate the button "Scheduled" and tap on the displayed time domain. In the following dialogue, you can define the hours of the day you want this mode to go into effect. The setting "Allow calls from" might be more meaningful with the selection "Favorites". You will only receive calls from those whom you have classified as "Favorites" in your contacts when the DnD mode is active.