STRESS MASKS CONSTANT-ROUTINE CORTISOL RHYTHMS J.W.T. Dekker 1 , H.P.A. van Dongen 1 , J.H.M. Souverijn 2 & G.A. Kerkhof 1 1 Dept. of Physiology, University of Leiden, The Netherlands 2 CKCL Dept., Academic Hospital of Leiden, The Netherlands INTRODUCTION Overt circadian rhythms often are perturbed by influences from an individual’s lifestyle and environmental factors. Clearly, these exogenous, so-called masking influences must be distinguished from those emanating from the endogenous circadian oscillator. Therefore, methods have been sought to take account of these masking influences (Minors & Waterhouse, ’89). Among these methods, the constant-routine protocol is becoming increasingly popular. The constant-routine protocol is purported to minimize the confounding effects of environmental (light/dark) and behavioral (sleep/wake) stimuli by confining the subject to a labo- ratory bedroom for at least 24 hours while keeping him/her awake, and maintaining a constant ambient temperature and illumination level. These very same efforts that are meant to prevent masking, however, may introdu- ce their own unwanted side-effects. Sleep deprivation and the ensuing stress, in par- ticular, can be expected to affect those physiological variables which are known as strong markers of circadian rhythmicity, i.e. core body temperature and cortisol secretion (Bonnet, 1994; Kuhn et al., 1969). Moreover, the occurrence of stress has been reported to influence circadian rhythmicity (Wever, 1979). This short paper presents the results of a preliminary analysis of urinary cortisol data, collected during two constant-routine recordings of six subjects, the first in March and the second in June 1996. In addition, from these same subjects urine samples were collected during a 24 h period under natural sleep/wake conditions. The main purpose of this analysis was to apply a model to the cortisol data which explicitly takes account of a potential stress factor. This was accomplished by incor- porating a linear term into the model which was applied to the data. METHODS Subjects and procedure. In March and June 1996 six young, healthy males (age 17 - 19 yrs) were recorded in a sleep laboratory under constant-routine conditions. Starting at 11:00 hrs, the subjects were kept awake in a near supine position in bed, in con- stant ambient temperature (21 °C) and constant illumination (100 lux), for a total duration of 26 hours. The first two hours were considered as an adaptation period, and not included in the analyses. Subjects were occupied with activities such as rea- ding, watching video movies, and casual conversation. Their daily nutritional and liquid intake was divided into hourly equi-caloric quantities. In addition, prior to the constant-routine of June, urine samples were collected during a 24 h period under natural sleep/wake (’field’) conditions. 43