Cortisol, often called stress hormone, follows a 24-hour cycle, aligning with circadian rhythms. It begins to rise during latter half of sleep, helping to stimulate wakefulness, said Dr Yatin Sagvekar, Consultant Neurology. Cortisol production is regulated by HPA axis, which operates at low levels during evening to prepare body for sleep. However, disturbances to HPA axis can alter this balance.