Stress makes it hard to fall asleep and stay asleep throughout the night. Like everything about sleep, different people respond differently, and some have little sleep disturbance from stress. Some may actually appear to sleep more, though doctors would probably describe this more as a symptom of depression or avoidance anxiety than stress. Generally speaking, stress is linked to shorter and lesser sleep than normal.
The causal relationship works both ways. People who are deprived of sleep usually show the signs of stress. Sleep deprivation causes stress on the body and mind.
The effect of stress on sleep duration and quality may be a reflection of how people cope with stress. Those who allow the emotions and anxiety to be foremost in their minds are more apt to suffer insomnia due to the stress than those who take a more rational and pragmatic approach.
Stress puts the mind on alert – hypervigilance (hyperarousal). The mechanisms for sleep are still there, but are overwhelmed by too-strong a waking impulse.
One marker of stress is elevated levels of the hormone cortisol in the blood. Cortisol is a way the brain signals to the rest of the body that the period requites a stress response. Levels of cortisol rise during the night. This is why blood pressure readings are higher in the morning than the evening.
One interesting connection is the one between cortisol and sleep architecture. People with the most fragmented and troubled insomnia sleep tend to produce a lot of cortisol in the evening the night. This suggests the cause of this sort of insomnia is not a too weak drive to sleep, but a too strong waking instinct – a hyperarousal.
Scientists have done experiments where they injected cortisol into men every hour during the night. The subjects had less REM sleep and more stage 2 and 3 sleep.
Cortisol administered early in the night or before sleep increased release of growth hormone. Later injections of cortisol did not affect growth hormone, but this is consistent with the fact that growth hormone is released as a pulse early in the sleeping period. These findings are consistent in both young adults and elderly men. Given that aging leads to lower levels of slow-wave sleep and growth hormone, the fact that elderly men respond to cortisol by increasing levels of both is interesting, but it doesn’t prove anything.
Another hormone involved in the stress response is corticoliberin and it has been shown that middle-aged and old men are more responsive, physiologically, to this hormone than young men. This aligns with observations of troubled sleep with aging and also supports the idea that this decline in tendency to sleep is due more to hyperarousal than to a decline in the need or drive to sleep. In tests where both young and middle-aged men are given shots of corticoliberin the young men are able to shrug it off and sleep soundly while the middle-aged men are stuck with insomnia.
"I love sleep. My life has a tendency to fall apart when I'm awake, you know?" - Ernest Hemingway
The stressed body produces more corticotrophin-releasing hormone which scientists have linked with mechanisms likely to cause insomnia. Stress (or more precisely fear) make the amygdala more active and reduces the production of sleep-promoting neurochemicals.
Norepinephrine levels tend to go up with Stage 1 sleep and down with slow-wave sleep. It has also been found that insomniacs have higher levels of cortisol in their urine.
Why does stress disturb sleep in some people more than others? A study of twins concluded that sleep reactivity to stress has a "substantial genetic component" and environmental effects.
PTSD is an anxiety disorder with many causes. It is frequently diagnosed in people who have had short periods of intense stress. Among the symptoms are those involving sleep, waking, arousal, and vigilance. Patients often have trouble getting to sleep or staying asleep. Some have trouble concentrating and some are hypervigilant. Time spent in REM is often higher in PTSD patients. The Department of Veteran Affairs has a webpage on PTSD and sleep.
Burnout is a term used both in popular parlance and, starting in the 1970s, in medical literature to refer to exhaustion and ennui that does not abate in periods of normal recovery (weekends, vacations). Prolonged stress causes burnout. If stress is the wear and tear on the body, sleep is a period when the body recovers from that wear and tear. But people with burnout do not recover from sleep, and frequently suffer chronic sleep disturbances.
People with burnout usually report sleep problems, if only excessive daytime drowsiness, and polysomnographic diagnosis shows fragmented sleep. Sleep duration is lower in those with burnout, and unlike regular stressed people without burnout, weekend naps do not solve the feelings of persistent sleepiness. Burnout patients do not seem to have much of a sleep debt, in the sense of a drive to sleep.