Receptional - 3:18 pm on Jan 4, 2013 (gmt 0)

>the impressions to different pages of the site during this interval are around the same

So do you mean that the surge was to ONE page or to ALL the pages? If one page, then I'll go with Mack's guess. You can use a backlink checker to see if you got new links - but more likely put your domain into a twitter search and see if people inked to it. If it is all pages then I would expect a spider and I am assuming that you are looking at raw logs rather than logs generated from a script on your page. Spiders visiting your site is generally a GOOD thing, because the content usually then ends up being indexed somewhere, somehow, but if you only have a few visitors a day, then a spider can seem big, because the behaviour is unusuay compared to a human.