Introduction: Circadian rhythms are biochemical, physiological or behavioural processes expressed in a temporal manner of approximately 24 hours. For the circadian system to function properly it must be entrained every 24 hours by exogenous time cues, also know as zeitgebers. Light is the major zeitgeber, which entrains the master clock. The central pacemaker is located in the suprachiamatic nucleus of the hypothalamus. It generates and regulates the circadian rhythms. Other biological clocks are located through out the entire body and these are all entrained by the SCN. Thereby, changes to the central pacemaker can be traced in the peripheral tissues. Disturbances of the rhythms are for instance found in sleep disorders and depression. The disturbances might be expressed by altered sleeping patterns, dysregulation of hormones like cortisol and melatonin, changes in body temperature ect.

Aim: The aim of this study is to investigate whether difference in sleeping patterns, also referred to as chronotypes, have different expression of melatonin and cortisol.

Method: 12 healthy volunteers were recruited for this study. The subjects were divided into two groups according to their chronotypes; morning or intermediate chronotypes. The chronotypes were defined, based on a questionnaire by Horne and Östberg. The study was divided into two different sessions; A pre-laboratory session where the subjects were asked to wear an actigraph for 5 consecutive days to validate their sleeping patterns; And a laboratory session where the subjects were admitted at the hospital. Each hour a blood sample was taken and in the morning saliva samples were taken. Plasma melatonin, plasma cortisol, and salivary cortisol concentrations were quantified by ELISA.

Results: No significant difference in the circadian rhythms of cortisol and melatonin were observed between the two groups of chronotypes. However, an amplitude reduction and phase delay in the circadian rhythm of melatonin was observed in the intermediate type profile group compared to the morning type profile group.

Conclusion: Further study with larger sample size is needed to investigate whether an actual difference between the chronotypes exist and whether one chronotype is more prone to develop depression than the other.