What is the sleep study that showed that a percentage of humans have a biological clock that is ~28 hours or greater? There was an article published some years ago about a sleep study (I think from Germany) that showed about 10-20% of people (European's I think) are genetically predisposed to function on a 30-hour* cycle. (As opposed to the 24 hour Circadian rhythm.)

*I think it was 30 hours, but don't know for sure. And I can't seem to find any reference to the study or what this "sleep disorder" is called. From Googling, I don't believe it is any of the following:


Delayed sleep phase disorder
Segmented Sleep Disorder
Non-24-hour sleep-wake disorder


The study was most likely related to cognitive function and may have been funded by a space agency.

Hope this is on topic here, I just saw this site in Area 51 but didn't read much about it.
Free-running circadian periods were estimated to vary from 13 to 65 hours in normal subjects according to a 1979 study by R.A. Wever. This paper was referenced in a more recent article regarding the circadian pacemaker (Czeisler et al., 1999). I could not access the Wever article directly to assess quality. However, the Czeisler article takes issue with the methods of earlier work that purported to show circadian rhythms longer than about 24 hours: 


  ...the generality of these
  findings has been limited by reports that activity (16, 17), knowledge of time of day (18), and
  exposure to ordinary indoor room light (19, 20)
  can shift circadian phase or alter the observed
  free-running circadian period in humans and
  thus may have influenced those observations
  (21)." 


Czeisler et al. sought to control these confounds and estimate a rhythm with very little variance centered at 24.18 hours.


  Czeisler, C. A., Duffy, J. F., Shanahan, T. L., Brown, E. N.,
  Mitchell, J. F., Rimmer, D. W., … Kronauer, R. E. (1999). Stability,
  Precision, and Near-24-Hour Period of the Human Circadian Pacemaker.
  Science, 284(5423), 2177–2181. doi:10.1126/science.284.5423.2177 PDF
  
  R. A. Wever, The Circadian System of Man: Results of Experiments Under
  Temporal Isolation (Springer-Verlag, New York, 1979); Experientia 40,
  1226 (1984).