Cortisol is a key player in the sleep-wake cycle, peaking in the morning to promote wakefulness before naturally declining throughout the day to induce sleepiness. Stress stimulates the hypothalamic-pituitary-adrenal (HPA) axis, which functions as part of both the nervous and the endocrine system, to release excess amounts of cortisol. Such overactivation of the HPA axis can heighten physical and mental arousal at night, disrupting sleep. While one night of poor sleep is unlikely to cause serious harm, frequent exposure to stress and excess cortisol can lead to serious health problems over time. (Poor sleep can itself increase levels of cortisol, which may perpetuate a cycle of heightened stress and poor sleep.)