How does daily amount of sleep vary within and between healthy adults?
Just a starting point.
Jean-Louis et al (2000) provide one sample of population adult sample using objective sleep measures. To quote the abstract


  Using Actillumes, we investigated whether self-reported sleep
  durations were indicative of a population decline in sleep duration.
  We also explored illumination and activity patterns. Methods: San
  Diego adults (n = 273, age range: 40–64) were recruited through random
  telephone calls and were monitored at home while engaging in usual
  daily routines. Results: Volunteers slept an average of 6.22 hours and
  received an average of 554 lux (environmental illumination). The
  timing of sleep, illumination, and activity occurred at 2:44, 12:57,
  and 13:43, respectively.


References


Jean-Louis, G., Kripke, D.F., Ancoli-Israel, S., Klauber, M.R. &amp; Sepulveda, R.S. (2000). Sleep duration, illumination, and activity patterns in a population sample: effects of gender and ethnicity. Biological psychiatry, 47, 921-927.