Medium-term effects of polyphasic sleep on performance Typical sleep patterns of one big block of 6 to 9 hours with no naps is usually referred to as monophasic sleep. A second natural sleep pattern is biphasic sleep which breaks up your sleep into two chunks (Wehr, 1992; Ekirch, 2001). My question concerns polyphasic sleep which partitions your sleep in 3 or more periods during the day.

From personal experience (and anecdotal evidence; this is a popular blogging topic), I know that the first one to two weeks of trying to keep a difficult polyphasic sleep schedule (say uberman that divides your day into 6 evenly spaced half-hour naps) produces very low performance. At first you are unable to fall asleep and you need your alarm clock to wake up and you feel groggy, agitated and slow. However, after the first week, a lot of these negative effects seem to disappear as you grow accustomed to the schedule. You start to get very vivid dreams during your naps (and I started to wake without the need for alarm clock) suggesting REM sleep. However, I have never subjected myself to proper performance tests while attempting polyphasic sleep.

What are the medium term (more than 2 weeks, less than a year) effects of polyphasic sleep on performance of tasks like working memory, reading speed and comprehension, and IQ tests?

Unfortunately, most sleep studies are less than two weeks in duration and report the obvious negative effects; except on working-memory which benefits from naps. Alternatively, they are military studies that only track vigilance (not having micro-sleeps)(Porcu et al., 1998; Rhodes &amp; Gil, 2003) , or have physical measures of no interest to me (Stampi, 1989).
Disclaimer: As you have noted yourself, there aren't very many scientific researches on the topic. The only main points that I can derive are generally of blogs or sketchy speculations. As such, you are supposed to take this answer with a grain of salt.

Interesting Thing of the Day notes that polyphasic sleep may make the person awake and alert but have adverse effects on the creativity and mental agility. 


  ...it may be that although one can indeed be awake and alert for 22 hours per day, creativity and mental agility suffer as a result.


Supermemo has the following to say about Polyphasic sleep:


  Yet you will definitely not want to sleep polyphasically if:
  
  
  you want to maximize your creative output
  you want to maximize your peak alertness, your average alertness, or minimize the impact of your worst alertness levels
  you want to maximize the health effects of sleep, etc.
  


While both the sources are not scientific findings, it does send a signal that polyphasic sleep cycles would have a negative impact on IQ tests and general activities like reading because your overall alertness is down because of sleep deprivation. Poor mental agility would have result in a lower working memory as well.