No, it's not the post-party hangover - your friends' New Year greetings and best wishes sent via SMS, email and iMessage to your iPhone, iPod touch or iPad may have been silenced despite your iOS Do Not Disturb setting being outside its scheduled time. In what appears to be a repeat of the old iPhone clock bug, some people reported their devices misbehaving on January 1, 2013, leading to muted alerts for incoming calls, messages and more.
Depending on your settings, the Do Not Disturb feature should have probably been automatically disabled on New Year Day's morning. The problematic behavior hasn't been observed by everyone, though it is surprising Apple hasn't sorted out time-related bugs in iOS fully yet...