Sleep Quality Over Time

Every morning I log how I felt my sleep was. I log it on a scale of 1 to 5, with a "3" being "meh" sleep, "1" is awful and "5" is something I've never experienced. I've been doing this daily since September of 13 - so about 6 months of data at this point.

Here's a plot of my sleep ratings computed with a 7 day running average - a 7 day boxcar filter, as it were. The raw data is difficult to visualize since it's a very quantized value.

That seems incredible to me that there's such a strong periodicity to my perceived sleep quality. Any ideas why? 

And in case someone thinks the window is the problem, here's the day with weekly averages plotted.