Seconded. Whenever I look at the graphs by Sipa, I cannot help imagining there must be some daily/weekly trends. For example, workplace computers being turned on and off daily. However, that would only be noticeable if the computers are unevenly distributed by timezone, although weekends would show a clear difference. Another weekly factor is that exchanges are quieter on weekends, when there are no bank transfers.
I wonder what it would look like if the hashrate curves for each week were normalized, so as to compare them to each other to spot any trends. This would also filter out most of the inherent variance in hashing results.