Journal of Psychosomatic Research 62 (2007) 595 – 596
Letter to the Editor
Further support for higher salivary cortisol levels in bmorningQ compared to beveningQ persons Brigitte M. Kudielka*, Silja Bellingrath, Dirk H. Hellhammer Department of Theoretical and Clinical Psychobiology, University of Trier, Trier, Germany Received 5 February 2007; received in revised form 5 February 2007; accepted 1 March 2007
Abstract We analyzed free cortisol daytime levels in a b3-days-7samples-a-dayQ protocol in relation to morningness in 112 middle-aged male and female adults derived from the first year of data of the Trier Teacher Stress Study using a one-item proxy for the assessment of chronotype. First, log-transformed cortisol levels were analyzed by a repeated-measures approach controlling for relevant covariates. Results yielded an almost significant main effect of morningness ( P = .06). Subsequent within-day analyses,
additionally controlling for awakening time, resulted in (marginally) significant main effects of morningness at Day 1 ( P = .06) and Day 3 ( P = .05) and a significant interaction of sample-bymorningness at Day 2 ( P = .04). In sum, the present data seem to corroborate the idea of higher daytime cortisol levels in morning relative to evening types in a sample of healthy middle-aged male and female adults. D 2007 Elsevier Inc. All rights reserved.
Keywords: Morning/evening type/person; Morningness–eveningness; Salivary free cortisol; Hypothalamus-pituitary-adrenal (HPA) axis activity; Chronotype
Many physiological circadian rhythms like body temperature, blood pressure, and heart rate appear to differ between morning and evening chronotypes showing earlier and/or higher peaks in blarksQ compared to bowlsQ [1,2]. Since the circadian rhythm of body temperature is generated by the hypothalamic suprachiasmatic nucleus, which also serves as a pacemaker for adrenocortical cortisol secretion, it has been hypothesized that the individual chronotype might also be an important source of interindividual variability in the circadian course of cortisol concentrations. To date, there is still a paucity of data investigating basal hypothalamus– pituitary–adrenal axis activity in terms of daytime cortisol levels in morning vs. evening chronotypes. Therefore, we examined salivary cortisol daytime profiles in a sample of 35 male and 77 female middle-aged healthy adults between 25 and 63 years (mean ageFS.D., 45.8F9.2 years) derived 4 Corresponding author. Department of Theoretical and Clinical Psychobiology, University of Trier, Johanniterufer 15, 54290 Trier, Germany. Tel.: +49 651 201 2981; fax: +49 651 201 3690. E-mail address:
[email protected] (B.M. Kudielka). 0022-3999/07/$ – see front matter D 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jpsychores.2007.03.016
from the first year of the Trier Teacher Stress Study who followed a b3-days-7-samples-a-dayQ protocol (samples: 1, 30, 45, and 60 min after awakening, 11 a.m., 3 p.m., and 8 p.m. on each day; two work days, and 1 leisure day). Volunteers with endocrine diseases (e.g., diabetes), medicated with corticosteroids or psychotropic drugs, were not eligible and subjects noncompliant to the saliva sampling protocol were excluded with the help of electronic monitoring [3]. A one-item proxy was applied for the assessment of morningness–eveningness. Following yes/no responses to the item bSind Sie ein Morgenmuffel?b (in German language; best translated as bI am . . .a morning mope(r)/. . .always grumpy in the morning/. . .not a morning person)Q 25 subjects (22%) were considered as bmore evening typeQ and 87 (78%) as bmore morning type.Q The observed distribution might be due to a selection process in the teaching procession. An overall general linear model (GLM) based on log-transformed cortisol levels controlling for sex and smoking status yielded a significant main effect sample ( F[4.2,386.8]=194.0, Pb.001) documenting the expected
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B.M. Kudielka et al. / Journal of Psychosomatic Research 62 (2007) 595 – 596
circadian cortisol change over the course of the day and a significant main effect day ( F[2,184] = 3.59, P = .03). The main effect type almost reached the level of significance ( F[1,92] =3.60, P =.06) indicating marginally higher cortisol levels in bmorningQ vs. beveningQ persons. Day-specific GLMs additionally controlling for individual awakening times rendered an almost significant main effect type at Day 1 ( F[1,92]=3.56, P = .06) and a significant main effect type at Day 3 ( F[1,90] = 3.89, P =.05), documenting higher cortisol levels in bmorningQ types. At Day 2, a significant interaction type by sample emerged ( F[4.6,406.8]=2.38, P = .04; higher cortisol levels in bmorning personsQ at the first six time points and slightly lower levels at 8 p.m.; main effect type: F[1,88]=1.35, P =.25). The present observation is in line with the previous reports rendered in predominantly young adults and males on salivary cortisol [4– 6] and total plasma cortisol [2,7]. There is one contradictory study in elderly subjects reporting lower cortisol levels and steeper slopes in subjects who were bmore morning people,Q as assessed by interview [8]. Though, this study as most of the others did not control for important covariates (like awakening time). Notwithstanding the fact that only a one-item proxy had been used in the present data (apparently adding a lot of noise that potentially blurs group differences if they really exist), we observed type differences in free cortisol daytime levels. In sum, the present data support the idea that bmorning personsQ have higher free cortisol values compared to bevening personsQ (a) applying an enlarged b3-days-7-samples-a-dayQ saliva sampling protocol, (b) excluding noncompliant subjects, (c) controlling for relevant covariates like awakening time, and (d) comprising a sample of 112 healthy male as well as female adults, extending generalizability. With this, being a
morning/evening person might be a relevant source of interindividual variability in adrenocortial activity.
Acknowledgments This study was supported by Emmy Noether research grant KU 1401/4-1 and KU 1401/4-2 of the German Research Foundation (DFG) awarded to Brigitte M. Kudielka.
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