Few studies focus on peripheral biological markers of personality. Melatonin (MLT), the main hormonal product of the pineal gland, has been used both as a diagnostic and therapeutic element and has been related with chronotype, depression and schizophrenia, among other psychiatric conditions. However, there is a paucity of studies on its use as a personality marker. The present work aims to determine whether serum MLT levels are related with the Eysenck's personality extraversion/introversion (E/I) dimension.

A sample of 100 healthy volunteers participated in the study. The E/I personality dimension was evaluated using the EPQ-BV (Eysenck Personality Questionnaire – Brief Version). Three blood samples were taken (at 09:00, 12:00 and 00:00 h) to measure MLT. MLT was analysed in serum by an ELISA. Serum MLT concentrations are expressed in pg/ml.

MLT levels displayed a clearly circadian pattern, with night levels being significantly higher than day-time levels (00:00: 35.78 ± 23.53 vs. 09:00: 7.78 ± 5.56, 12.00: 3.35 ± 1.94, p < 0.05). Serum MLT levels at 09:00 h were significantly higher than MLT levels at 12:00 h (7.78 ± 5.56 vs. 3.35 ± 1.94, p < 0.05). No significant correlations were found between E/I scores and serum MLT levels at 09:00 (r = – 0.06, p = 0.55) 12:00 (r = – 0.12, p = 0.25) and 00:00 h (r = – 0.01, p = 0.99).

There is no relationship between MLT levels and the E/I personality dimension. Future studies should examine the neuroticism dimension of personality.