Arylalkylamine N-acetyltransferase (AA-NAT) is a rate-limiting enzyme in 
melatonin hormone synthesis and participates in daily oscillations of the 
melatonin level. We studied the association between the AA-NAT gene and delayed 
sleep phase syndrome (DSPS). Results indicate that there is a significant 
difference in allele positivity at the single nucleotide polymorphism involved 
in an amino acid substitution from alanine to threonine at position 129 between 
patients with DSPS and healthy controls. The frequency of the 129 threonine 
allele is significantly higher in the patients than in the controls ( P=0.0029). 
The data suggest that AA-NAT could be a susceptibility gene for DSPS.