Proving once again that even the latest, greatest iOS time-based feature can fail in the face of simple change, tons of reports are flying around the internet that Do Not Disturb -- the feature that lets you mute Notification Center alerts during specified, usually nighttime, periods -- stopped working at some point between 2012 and 2013.
Manually flipping Do Not Disturb from off to on again seems to fix the problem for some people, rebooting for others, though that might be small comfort when the Game Center trumpet goes off at 4am after you've been out old lang syne-ing until the wee hours, or when it won't turn back on and you miss that important call.
Apple has struggled with alarms and other time-based functions during transitional periods before, and it seems they still haven't figured out a way to fix whatever's causing the problems.
I have almost all my alerts turned off all the time anyway, so I didn't notice the problem, but if you did, let me know what happened, and if you found a fix that worked for you.
Happy New Ye---BAPADABAAAAAAAAH!
Update: Apple has released a knowledge base article saying the bug will fix itself on January 7.