The Sunday Night Blues are ingrained into most Western World (and beyond) individuals from the early days of pre-school. This feeling of melancholy stems from the end of the weekend and the impending weight of the upcoming school/work week. It is one of the oldest constantly reappearing emotions we feel - an emotion that is nearly identical at age 6, at age 16, at age 26, at age 50, and probably all the way until retirement.
This unpleasant state begins as the sun sets on a regular (non-holiday enhanced) weekend. It fosters strange acts - i.e. giving in to bad television shows or indulging is unhealthy meals. It leads us to postpone sleep for as long as possible in order to prolong the freedom of the weekend. The emotion mutates into apprehensive submission and then finally into the eventual flow of a high-performing study/work week.
For those sufficiently crass and opportunistic, the Sunday Night Blues offer a chance to reach a different emotional state in target consumers. For instance, I suspect that vacation-themed advertisements might reach a more receptive and accepting audience on Sunday nights. But for those looking to make the world a happier place, the Sunday night timeslot offers a great chance for positive innovation and design - for the opportunity to create the solution to a major collective downer.