Melatonin, a major photoperiod-dependent hormone, regulates circadian rhythms 
and biological rhythms and acts as a prominent sleep promoter. Symptoms related 
to hypermelatoninemia have been reported in individuals supplemented with 
melatonin. However, spontaneous endogenous hypermelatoninemia has not been 
reported previously. A 6-year-old girl previously diagnosed with Shapiro's 
syndrome was admitted to our hospital on several occasions during a 1-year 
period with complaints of altered consciousness, syncope, hypothermia and 
episodes of sweating. The episodes occurred daily and during sleep and lasted 
for 1-6 h. During these episodes, she sweated profusely and felt faint and her 
skin was pale and cool. Other complaints included recurrent abdominal pain, urge 
incontinence and myopia. She was shown to have hypermelatoninemia (>1,000 pg/ml, 
normal range 0-150 pg/ml) during these episodes. The duration of her attacks 
decreased with phototherapy and she was successfully treated with propranolol. 
To our knowledge, this is the first case of hypermelatoninemia without any 
detectable organic pathology. We did not determine the exact mechanism of 
hypermelatoninemia in this patient; however, it might have been related to 
irregular control of pinealocytes by the suprachiasmatic nucleus or related 
pathways. Hypermelatoninemia should be considered in patients with spontaneous 
periodic hypothermia and hyperhidrosis, and also in patients with Shapiro's 
syndrome.