Amplitude reduction of plasma melatonin rhythm in association with an internal desynchronization in a subject with non-24-hour sleep-wake syndrome.

The plasma melatonin rhythm was measured longitudinally in a subject with non-24-h sleep-wake syndrome, and the amplitude and area under the curve (AUC) of the melatonin rhythm were investigated in relation to the sleep-wake cycle. When the melatonin rhythm and sleep-wake rhythm were internally desynchronized, the amplitude and the AUC were reduced significantly. These parameters were not influenced by external melatonin administration. These results suggest that a causal relationship between the reduction of circadian oscillation and internal desynchronization exists in this subject.