Nick mentions using Skritter (or spaced repetition in general) as a cognitive assay (in place of other cognitive tests like dual n-back) to figure out how well your mind is functioning at any given time.

However, I haven't been able to get this data usefully from Skritter, since Skritter aggregates your stats daily (rather than by session). Does anyone know how to get this data?

When I tried it, it turned out to not really give any data (and I had a lot of data), so either my self-report (to which I was trying to correlate it) or the Skritter data weren't very reflective of my cognitive performance. I had an export mode where it would grab the stats on each individual review and did some analysis on that.

I now think that cognitive assays should probably be very low level if you want to pick up on anything less obvious than massive sleep deprivation or drugs: http://www.quantified-mind.com/