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

The amplitude and area under the curve (AUC) of melatonin rhythm were significantly reduced when the melatonin rhythm and sleep-wake rhythm were internally desynchronized.

• The plasma melatonin rhythm was measured in a subject with non-24-h sleep-wake syndrome over time.
• Internal desynchronization between melatonin and sleep-wake rhythms was associated with reduced amplitude and AUC of the melatonin rhythm.
• External administration of melatonin did not affect these parameters.
• These findings suggest a potential causal link between reduced circadian oscillation and internal desynchronization.

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