It is hypothesized that one of the primary abnormalities of primary circadian rhythm disorder (PCRD) is the strong link between any episode of sleep and circadian rhythm. To test this hypothesis, the relationship between napping and responsiveness to hypnotics was examined in 12 patients with PCRD. A significant association was found (P = 0.04, chi2 test). Patients with PCRD who napped were all responders to hypnotics. The results suggest a strong link between episodes of sleep and circadian rhythm in some patients with PCRD, and might also suggest the heterogeneity of PCRD. Napping in patients with PCRD may be a predictor for responsiveness to hypnotics. In addition, napping and responsiveness to hypnotics might have a clinical value to differentiate PCRD from secondary CRD.