What is the sleep study that showed that a percentage of humans have a biological clock that is ~28 hours or greater?
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).