This is not so easy: we had candles and fun at nights for a long time, and while it's not clear how widespread it was, there were definitely periods in history

This is not so easy: we had candles and fun at nights for a long time, and while it's not clear how widespread it was, there were definitely periods in history when it was common to sleep in separate chunks. Social life is important, in fact most of the not-sleeping we're doing nowadays is due to social interaction and/or entertainment, not acquiring food. Furthermore, depression is has a very strong impact on survival and breeding chances[citation needed], so it shouldn't really need that much time to disappear unless there are reasons for it to stay there. But, there are attempts to figure out why exactly wasn't depression weeded out by now (other than the rather sad "because it's somehow inherent to other aspects of our brain and really hard and/or costly to avoid): http://en.wikipedia.org/wiki/Evolutionary_approaches_to_depression http://en.wikipedia.org/wiki/Evolutionary_approaches_to_depr...