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Getsuyuu Syndrome & The Moon
In minor cases of Luna Sedata, fragmentary memory loss and behavior similar to sleepwalking can be observed.
Characteristically, the symptoms vary with the waxing and waning of the moon.
When the moon is full, the patient's condition is stable, but there is a tendency to go outside and seek the moonlight. Around the new moon, the patient becomes more unstable and increasingly frightened.
Once, Luna Sedata was called "Moonwalking" or "Haunted Walking", which seems to be derived from that behaviour.
In severe cases, the patient loses almost all memory but remains attracted to the moonlight and seeks out window sills and rooftops.
Then, the patient bathes in the moonlight. More lucid patients describe the feeling as momentarily regaining something they've lost.
A person who has lost their memory will always look up to the moon.
Such an odd symptom can surely happen in a psychologically disturbed individual, but there is no explanation why a group of people would display the same symptoms.
Since this is a peculiarity of Rougetsu Island's cultural sphere, there must be a relationship to its beliefs and customs.