Relations between common nightmares and chronobiological factors remain poorly understood. The possibility that nightmare frequency and distress are associated with chronotype ("morningness-eveningness") was investigated in a sample of respondents to an Internet questionnaire. Over a 4(1/2)-year period, a total of 3978 subjects (mean age = 26.5 +/- 11.6 yrs; age range = 10-69; 2933 female, 1045 male) submitted responses to single items about chronotype and nightmares as well as to other demographic variables. Analyses of chronotype and nightmares items by age and gender replicated most previous findings for these measures-validating their further assessment-and uncovered abrupt increases in nightmare distress between ages 10-19 and 20-29 for females and ages 30-39 and 40-49 for males. Most important, there was a strong association between nightmares and eveningness for female subjects. The latter was expressed as a linear association between nightmare frequency and increasing eveningness and a cubic association between nightmare distress and increasing eveningness; the definite evening types displayed the most severe nightmares. The effect for nightmare frequency was independent of age and sleep duration but was eliminated when nightmare distress was covaried. For females, the nightmare/eveningness association appeared at ages 20 to 29 for the definite evening type and only later, at ages 30 to 39, for the moderate evening type. Findings are consistent with the possibility that nightmares are the expression of a more general pathological factor that is characteristic of eveningness and that is responsible for affective symptoms such as neuroticism and depression. This pathological factor appears to be expressed in late adolescence/ early adulthood, and relative morningness may be a protective factor delaying its onset. The well-established circadian modulations of cognitive, social, and affective tasks that are influenced by chronotype may extend to the memory and affective processes of sleep-including dreaming. This chronotypic influence, together with a likely gender difference in the neurophysiological substrate of emotional processing, may result in the differential occurrence of nightmares for female evening types.
