Chronotype as modulator of morning serum melatonin levels.

Background: The search for biological markers of individual characteristics has produced scanty results. Melatonin (MLT), the main hormonal product of the pineal gland, has been used as a biological marker of neuroticism, introversion-extroversion and morningness-eveningness. Morningness-eveningness indicates preferences associated with morning or evening activities. The goal of this research is to study if serum MLT levels are related to morningness-eveningness preference.

Methods: Twenty-three morning type and twenty-one evening type healthy volunteers took part in the study. Morningness-eveningness was evaluated with the Composite Scale of Morningness. Blood was drawn at 09:00, 12:00 and 00:00 h. MLT levels were measured with an ELISA.

Results: At 09:00 h evening type subjects had significantly higher serum MLT levels than morning type subjects (8.4±3.6 pg./ml. vs. 4.6±3.2 pg./ml., p<0.02).

Conclusions: Morning serum MLT may be used as a biological peripheral marker of morningness-eveningness preference. Our results emphasise the convenience of expanding MLT studies until 09:00 h when differences between morning type and evening type subjects may still be found.