Compared with good sleepers, people with insomnia secrete more cortisol in the evening before bedtime and in the first half of their sleep; they also have more fast brain-wave activity in non-rapid eye movement (NREM) sleep, both indicators of arousal. Stress is the principal cause of the insomnia that plagues so many millions of Americans.
How does stress cause insomnia? To understand that, we must first know a little bit about the physical processes that occur when we fall asleep. Falling asleep is a natural process that involves a distinct sequence of events in the body. As we approach sleep, there is a gradual lowering of metabolism. Our heart rate slows and our blood pressure declines. Our breathing becomes more regular, and we consume less oxygen. At the same time, there are changes in the processing activity of the brain. The activity of neurons in the cerebral cortex becomes first slower, and then more synchronized, indicating a shift away from the complex, activated patterns of waking consciousness and toward a homogenous, deactivated state. As a result, we cease to process the sensory messages coming in from the outside world, and we slip into quiet sleep.
The adrenal hormone cortisol is a trigger of the stress response. That's not cortisol's only job, however. In addition to the sharp transitory peaks of cortisol secretion that characterize the stress response, there is also a daily, cyclical rise and fall of cortisol levels that govern our level of wakefulness throughout the day and night. Cortisol is excitatory; it arouses us and wakes us up. Blood levels of cortisol have been shown to increase between 50 and 160 percent within thirty minutes of waking; that produces the powerful jolt of arousal needed to wake us up and get us moving in the morning. Then, cortisol levels should decline as the day wears on and reach their lowest point in the evening, allowing us to rest, relax, and sleep.
But, as we know, cortisol levels can also be affected by the conditions of our daily existence. Dangerous, demanding, or threatening events--stressors--cause us to temporarily secrete higher levels of cortisol. That's a good thing, because we need to be aroused in order to answer the challenges that arise in the course of our lives. But when, as a result of prolonged or unremitting stress, whether real or perceived, our cortisol levels get stuck at a chronically higher level, that's bad news for our bodies and minds, and especially bad news for our ability to sleep and rest. Chronic over-secretion of cortisol leaves us chronically hyperaroused. Numerous studies indicate that insomnia is accompanied by excessive activation of the stress-response system not only during waking hours but during sleep as well. Furthermore, chronically elevated levels of cortisol and its precursor, adrenocorticotropic hormone (ACTH), can make sleep shallow, fragmented, and unrestful; delay the onset of sleep; and produce more frequent nocturnal awakenings.
Stress is the principal cause of insomnia. Stress hormones are excitatory. When stress becomes chronic, we become chronically excited, or hyperaroused. When we're chronically hyperaroused, we can't sleep, and the sleep we do get is not as restful.