Stress brings destruction when a hormone called cortisol exceeds healthy levels in the body. Cortisol is present in the body constantly and follows a circadian rhythm. It helps you get up in the morning when it is the highest in its natural cycle. It also peaks a couple of times more throughout the day – in the early afternoon and early in the evening, however to a lesser degree.
Cortisol is released to help you perform an activity that requires a bit more of an effort. It is involved in metabolism, immune response, blood sugar, and blood pressure regulation. You need cortisol to survive, and too little of it is not healthy. But neither too much cortisol is healthy, and since it is released in response to stress, more often than not, modern homo sapiens suffers from cortisol excess.
When cortisol is involved in stress response, the body secrets more of it, adding to the levels which follow a circadian rhythm. Because cortisol works mobilizing on the body (preparing you to fight or flight), you may be experiencing difficulties falling asleep or remaining sleeping throughout the night. Not enough rest causes additional stress to the body, adding to already higher cortisol levels.