Insomnia is a very common sleep disorder, and presents in various types. However, sleep maintenance insomnia (SMI) is the most frequent complaint. Both the cause of awakening and the difficulty in returning to sleep are important in precipitating episodes of nocturnal sleep disturbances. Research seems to suggest that hyperarousal states, personality traits, and physiologically determined changes in sleep onset, all contribute to insomnia.
Risk factors
Nocturnal awakenings are more common in women, and increase in frequency in older age groups.
The following factors may help to bring on insomnia in individuals predisposed to it:
- Comorbidity with chronic medical illnesses increases the risk of insomnia
- Conversely, 75-90% of people with insomnia have a higher chance of having chronic conditions which cause pain, gastrointestinal dysfunction, or hypoxemia, as well as neurodegenerative disorders
- Psychiatric disorders are associated with a higher prevalence
- Menopause and puberty
- Shift work
- Primary sleep disorders such as periodic limb movement syndrome, sleep-disorder breathing, and restless limbs syndrome
- Disturbances of Circadian rhythm
Self-reported causes for awakenings
In one sleep study, self-reported causes for awakenings from sleep include:
- Need for bladder voiding (75%)
- Spontaneous waking
- Feeling thirsty
- Hearing a noise (these three accounted for 30-40%)
- Disturbance by children, co-sleepers, or pain (10-20%)
- Hunger pangs or breathing problems (5%)
It is important that objective assessment of the sleeping patients who awoke during their sleep usually revealed a preceding breathing disturbance while the patient still appeared to be asleep, and which was unobserved by the patient. This was despite the phenomenon being significant enough to fit the criteria for obstructive sleep apnea and upper airway resistance syndrome in most cases. This has been interpreted to mean that hyperarousal, either due to psychological or innate factors, or caused by the hypoxemia and hypercapnic conditions of sleep breathing disorder, makes the patient more susceptible to physiological or pathologic activation leading to clinical awakenings.
It is as of now impossible to explain nocturnal awakenings without any disordered breathing as a trigger. Yet it is evident that the difficulty in returning to sleep is as important as the cause of the awakening, in causing this type of insomnia. This is even more clear when it is shown that patients with classic sleep apnea but no indicators or complaints of insomnia showed a higher rate of awakenings, so that the apneic event is more of a catalyst rather than the actual cause of the insomnia.
Etiological factors
A range of factors has been studied for their role in SMI.
- Emotional and psychiatric disturbances
This may include the presence of life stressors, tense personality, and the presence of psychological illnesses. It is noticeable that individuals who awaken frequently have a higher level of frustration with life, unsatisfying relationships, poor ways of coping with stress, and low feelings of self-worth.
Psychiatric conditions including major depression and bipolar disorder are associated with higher prevalence of insomnia, and conversely, patients with insomnia have traits such as depressed mood and emotional inhibition. Thus, it needs to be determined whether poor maintenance of sleep is the cause or result of psychopathological conditions.
Primary sleep disorders
Conditions such as sleep apnea are not causative factors for insomnia.
The following are examples of conditions which cause insomnia:
- Any illness associated with pain
- Respiratory difficulty due to pulmonary congestion or airway stiffening
- Chronic renal disease
- Gastrointestinal disorders
- Thyroid conditions
- Neurologic disease
- Habits, addictions, and medications
Smoking and alcohol abuse, as well as the overuse of stimulants such as amphetamines, may cause insomnia
Drugs such as beta-blockers, selective serotonin-receptor inhibitors (SSRIs), and withdrawal from certain narcotic drugs, can cause insomnia
Internalization model
Insomnia is mostly found in people who repress and internalize their emotions, especially feelings of anger, leading to the hypothesis that such people are in a state of constant emotional hyperarousal. Associated with this, the physiological functions become activated, resulting in events unfavorable to sleep onset, such as increased heart rate and muscle tension, racing thoughts, and anxiety, with brooding over life events and possibilities. This hyperarousal state makes sleep resumption difficult if they wake in the middle of the night. This in turn breeds a fear of nocturnal awakening which adds to their aroused emotional condition and feeds the sleep difficulty. Thus, a vicious cycle of insomnia is formed. The three Ps (Spielmann) that comprise this model are:
- Predisposing characteristics that make a person prone to insomnia of a specific type
- Precipitating events preceding the onset of insomnia
- Perpetuating attitudes or habits which increase the brief waking interval common to the sleep of most individuals to a significant one which interrupts sleep in insomniacs with nocturnal awakenings
Thus, the insomnia may become independent of the initial stressor and act as an independent entity over time.
Evidences in support
In support of this hypothesis, the following data may be adduced:
- The lack of daytime sleepiness despite increased daytime fatigue, due to hyperarousal and increased alertness in patients with SMI
- The presence of markers of physiological activation in some patients with insomnia
- Changes in the EEG findings in insomniac patients such as a higher beta- and gamma-wave activity during NREM sleep, the lack of the normal steep drop in alpha waves, and increased beta- and alpha-waves during REM sleep
- Neuroimaging studies also show evidence of higher levels of brain metabolism because of persistent arousal states. There is also a lack of evidence of sleep deprivation as shown by cognitive deficits, in patients with insomnia, despite their self-reported lack of sleep.
- Markers of stress such as norepinephrine and corticosteroid metabolites are elevated in insomniacs, perhaps reflecting stress-induced activation of the hypothalamo-pituitary-adrenal (HPA) axis and the sympathetic nervous system.
- HPA axis activation could also explain the abnormal circadian rhythm of secretion of many pro-inflammatory cytokines such as TNF-α or IL-6 in these patients.
- In addition, middle age may make individuals more susceptible to the arousal effects of stress, accounting for the higher prevalence of insomnia in this age group.
References
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1978319/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2128619/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2830306/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3490361/
Further Reading
Understanding the pathophysiological relationship between OSA and metabolic syndrome