Self-report of circadian type reflects the phase of the melatonin rhythm

This study examined the relationship between circadian rhythm characteristi cs of the pineal hormone melatonin and individual differences in circadian type and mood. 95 healthy young men and 22 women were assessed each hour (0 0:00-07:00 h) for blood levels of melatonin throughout one night in the lab oratory. Each subject was assessed for circadian type (morning, afternoon, or evening type) and morning mood (PANAS). Circadian type was strongly rela ted to the melatonin acrophase but not to amplitude or time of year of asse ssment. Also, morning types evidenced a more rapid decline in melatonin lev els after the peak than did evening types. Evening types were younger than were morning types. Female morning types reported more positive affect upon waking than did female afternoon or evening types. Males showed no such di scrimination. Age was related to both melatonin acrophase and circadian typ e but did not explain the relationship between them. The results replicate and extend findings on circadian type and psychological and physiological v ariables. (C) 1999 Published by Elsevier Science B.V. All rights reserved.