Free‐running circadian rhythm of melatonin in a sighted man despite a 24‐hour sleep pattern: A non‐24‐hour circadian syndrome

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Abstract
Sleep and plasma melatonin rhythms were measured longitudinally in a sighted young man (21 years old) under a day‐night environment. At each measurement, the responsiveness of the melatonin rhythm to a single light pulse was examined in addition to the 24‐hour profile. In experiment 1, the timing of sleep was decided by the subject himself. Although most sleep episodes were observed between 21:02 h and 10:55 h, the plasma melatonin rhythm free‐ran for a period of 24.18 h. In experiment 2, the sleep‐wake schedule of the subject was strictly fixed. The subject was instructed to go to bed at 24:00 h and wake up, at the latest, before 8:00 h for 40 days. The melatonin rhythm, however, continued to free‐run for a period of 24.12 h. Nocturnal melatonin level could not be suppressed by a 3‐hour light pulse of 500 lx, but was suppressed by a pulse of 1000 lx. It is concluded that internal desynchronization occurred in this particular sighted subject where the sleep‐wake rhythm was entrained by the 24‐hour day‐night environment, whereas the plasma melatonin rhythm free ran, and that a forced sleep schedule did not act as a strong zeitgeber.