Subsensitive melatonin suppression by dim white light: possible biological marker of panic disorder

Light is involved in providing entrainment of circadian rhythms and the suppression of the pineal hormone melatonin. In patients with affective disorders, there have been indications of circadian as well as seasonal variation in illness, which may be reflected in melatonin production. Varying sensitivity to light has been noted within healthy individuals as well as in some patients with affective disorders. Recent evidence suggests that patients with panic disorder may have an altered and phase-delayed melatonin rhythm. The present study examined the nocturnal plasma melatonin rhythm in patients with panic disorder, and also examined their melatonin sensitivity to dim light. The melatonin rhythm was examined in 6 patients with panic disorder and 8 controls. The melatonin sensitivity to dim white light (200 lx) was examined in 8 patients with panic disorder and 63 controls and was compared to that of a group of 7 patients with other anxiety disorders. Patients with panic disorder demonstrated a trend towards higher and delayed peak melatonin levels compared to controls. Patients with panic disorder also had a subsensitive melatonin suppression by dim white light, compared to controls and patients with other anxiety disorders (p<0.005). The phase-delayed circadian rhythm observed in patients with panic disorder may be secondary to the subsensitivity of the melatonin response to light. It is hypothesized that the subsensitivity may be due to abnormal neurotransmitter/receptor systems involved in regulation of melatonin suppression and circadian rhythmicity, and may lead to phase- delayed circadian rhythms. The melatonin subsensitivity to light may be used as a biological marker of panic disorder.