Variation of length of circadian rhythm in humans in absense of time reference
This might be the study to which you refer in your answer, the people tested averaged a daily cycle of 24 hours and 11 minutes with a surprisingly small variation in their rhythms:


  The variation between our subjects, with a 95 percent level of confidence, was no more than plus or minus 16 minutes, a remarkably small range.


Worth noting they were all healthy and so it doesn't tell us much about insomnia.

This study about insomnia in adults with ADHD shows that circadian rhythms of insomnia sufferers aren't so much longer as delayed (for sleep-onset insomnia anyway). Figure 2 might be of interest, showing that insomnia sufferers have significantly delayed melatonin production.