Psychological stress and sleep

Stressors can be categorised into the Challenge/Hindrance stressor model. Challenge stressors, while unpleasant, allow for growth and achievement such as time pressure in a work context. Hindrance stressors are those that cause unnecessary burden and do not contribute to achievement such as poor work supervision.[7] Self-reported quality of sleep reduces in relation to more hindrance stressors but not to challenge stressors. Sleep duration does not change in relation to the hindrance: challenge ratio and neither does increased stressful working hours effect self-reported sleep quality.[7]

Insomnia is a common condition affecting up to a fifth of the population in many countries across the world and is often complicated by a number of psychiatric conditions. Paradoxical insomnia is the phenomenon of a discrepancy between reported sleep duration and objective measurement of sleep. In some cases however the stress and anxiety produced does, in fact, result in an objective reduction in quality of sleep.[5]

Sleep can be broadly split into the lighter "rapid eye movement" (REM) and deeper "non-rapid eye movement" (NREM).[3] Changes in sleep phase varies in animal models depending on the stressor but does not alter total sleep duration with the exception of novelty which reduces both REM and NREM. Conditioned fear for example reduces REM sleep whereas auditory stimulation increases it.[3]

In humans, models of stress have been closely linked to the context depression. Changes in sleep patterns in depression are very close to those seen in acutely stressed animals; these changes can be used as a predictor for developing depression. Long-term/ chronic psychosocial stress is known to cause depression symptoms but the effect of chronic stress on sleep without the confounding factor of depression is unclear.[3] Early life sleep disruption caused by stressors may affect neuroplasticity and synaptic connectivity potentially leading to the development of mood disorders. This poor sleep may become a stressor itself compounding the phenomenon.[6]

Observations have been made that there is an association between stress, sleep and Interleukin-6 proposing a possible mechanism for sleep changes.

During both chronic and acute phase sleep deprivation there are increases in the pro-inflammatory cytokine Interleukin-6 (IL-6). Not only is IL-6 influenced by the circadian rhythm but its effectiveness is increased by sleep itself as there is an increase in serum IL-6 receptor during sleep. After periods of long sleep deprivation the first post-deprivation sleep shows a marked drop in IL-6 and an increase in slow wave sleep / "deep sleep". Similarly napping during the day time has been shown to decrease IL-6 and reduce tiredness.[8] When humans are injected with exogenous IL-6 they display an increase in fatigue and other sickness behaviour.[8]

This IL-6 increase is also observed during times of increased psychological stress. In a laboratory setting, individuals exposed to psychological stressors have had raised IL-6 (and acute phase protein CRP) measured especially in those who displayed anger or anxiety in response to stressful stimulus.[8] Just as the human body responds to inflammation-inducing illness with increased fatigue or reduced sleep quality, so too does it respond to psychological stress with a sickness behaviour of tiredness and poor sleep quality. While sleep is important for recovery from stress, as with an inflammatory illness, continuous and long term increases of inflammatory markers with its associated behaviours may be considered maladaptive. [8]

Since the American Civil War there has been multiple "war syndromes" reported such as 'irritable heart', 'effort syndrome' and 'Gulf War Syndrome'. Thought to be discrete and different from post-traumatic stress disorder (PTSD), these war syndromes have a range of physical symptoms but commonly feature sleep disturbances, tiredness, poor concentration and nightmares. The historic picture is unclear due to poor contemporary understanding of psychological illness and, in more modern conflicts, gathering data has been difficult due to operational priorities; no cause has been identified that isn't connected to psychological stress.[9]