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.