Spline Models for Cortisol Circadian Rhythms

• Activity Number: 193
• Type: Contributed
• Date/Time: Monday, August 10, 2015 : 10:30 AM to 12:20 PM
• Sponsor: Biometrics Section
• Author(s): Jacob Wegelin* and Kirk Warren Brown
• Companies: Virginia Commonwealth University and Virginia Commonwealth University
• Keywords: cortisol ; stress ; cubic spline ; longitudinal data ; clinical trial

Abstract

Human salivary cortisol is often used as an indicator of stress, both in case-control studies and in randomized trials. Samples can easily be taken by study participants at home, for later lab analysis. The most effective indicator of stress is probably not an individual's average level, but rather the shape of the trajectory throughout the day. In a healthy individual, cortisol follows a wave-like pattern: It peaks a half hour after we get out of bed and subsequently declines over the course of the day. Consequently, salivary cortisol samples are often taken longitudinally, 3 to 6 times per day. Many studies, however, have not accounted for differences in trajectories, but rather rely on an overall summary such as area under the curve (AUC). We will demonstrate how cubic splines can be exploited to characterize the shapes of cortisol trajectories and to test for differences between groups.