The most cited paper on this is "Stability, Precision, and Near-24-Hour Period of the Human Circadian Pacemaker" (Czeisler, et al., 1999), which found that thei

The most cited paper on this is "Stability, Precision, and Near-24-Hour Period of the Human Circadian Pacemaker" (Czeisler, et al., 1999), which found that their 24 subjects between ages 13 and 65 had a mean circadian rhythm of 24 hours 10 minutes with a standard deviation of 8 minutes. Earlier studies were affected by artificial light lengthening cycles. Non-24 sleep-wake disorder in sighted people is not researched well enough to have accurate rates: there are only about a hundred cases in the existing studies and the disorder is not well known and probably underdiagnosed.