Assessing the Influence of the Inner Clock on the Cortisol Awakening Response and Pre-Awakening Movement

Our inner clock is responsible for creating a circadian rhythm that controls our sleep-wake cycle, and, hence, is also involved in the process of awakening from sleep. Awakening is accompanied by the cortisol awakening response (CAR). The purpose of the CAR is, presumably, to prepare our body for the upcoming challenges of the day. It is assumed that our inner clock is anticipating awakening, and thus, initiates the awakening process while we are still asleep, leading to the common phenomenon of waking up immediately before a known alarm. However, the role of the inner clock on the awakening process was only assessed in sleep laboratories and using invasive, blood-based biomarkers. For that reason, we investigated n = 117 participants by collecting cortisol data from saliva samples and IMU data from a wrist-worn inertial measurement unit (IMU) sensor node in a home environment over two nights. We compared cortisol data, characterizing the CAR, and IMU features, characterizing pre-awakening movement, between spontaneous awakening, awakening by a known alarm, and by an unknown alarm. We observed significant differences between the three study conditions in both cortisol and IMU data indicating higher cortisol reactivity and less movement if participants woke up by an unknown alarm. Our findings all support the assumption that our inner clock is anticipating our wake-up time. Utilizing our results, this work lays the foundation for the development of automatic classification models aimed at determining the ideal awakening time of individuals based on the analysis of pre-awakening movement.