What Causes Insomnia?
In many cases, insomnia may be a consequence of another underlying medical problem.
Mental Health Issues
Insomnia is a symptom of many mental health problems, including anxiety, depression and bipolar disorder (Morin 2006; Buysse 2005; Baroni 2012).
Not only can mental health disorders trigger insomnia, but insomnia can be a major risk factor for mental health issues. Data indicate that insomnia complaints are a major predictor for onset of depressive disorder within 1-35 years (Buysse 2005).
Insomnia is also linked to certain psychological personality traits, such as social introversion and repression of feelings (Singareddy 2012).
Psychophysiological insomnia (PPI). PPI, a type of chronic insomnia, is associated with excessive worrying specifically focused on not being able to sleep. It appears to be linked to hyper-arousal when going to bed (Sato 2010; Bonnet 1997; Bastien 2008). The hypothesis behind it is that afflicted individuals have a hard time relaxing and settling down when they go to sleep, resulting in "racing thoughts." They then focus on their difficulty falling asleep, which results in anxiety that further disturbs sleep. Over time, poor sleep and worrying about sleeping can become associated with going to bed, resulting in a pattern of chronically poor sleep that affects daytime activities. Some believe that in addition to heightened arousal, individuals with PPI may have some dysfunctional neurological inhibitory mechanisms that would normally help the mind "dis-engage" from daytime thought patterns (Espie 2002), preventing them from falling asleep.
Physical Health Issues
Many conditions are associated with insomnia, including musculoskeletal problems, cardiovascular disease, gastrointestinal and urinary problems, neurological problems, respiratory problems, immunological problems, and cancer (Sivertsen 2009; Buysse 2005; Taylor 2007; Geyer 2008; Katz 1998; George 2000).
Levels of sex hormones (i.e., estrogen, progesterone, and testosterone) may have a significant impact on sleep. This is especially true for women; the incidence of sleep disturbances in women rises to 40% three years after menopause (Woods 2005). Studies have found that hormone replacement therapy in menopausal women can significantly improve sleep (Silva 2011; Saletu-Zhylarz 2003).
The relationship between sleep and hormone levels occurs in men as well; lower levels of testosterone correlate with increased severity of obstructive sleep apnea (a particularly serious sleep disorder) (Hammoud 2011). People with trouble sleeping should have their hormone levels tested. It used to be thought that higher testosterone levels in men worsened sleep apnea, but more recent studies show it is low testosterone that is associated with sleep disturbance s in aging men (Barrett-Connor 2008; Canguven 2010).
Medication-induced insomnia can be caused by a wide variety of drugs, including decongestants, monoamine oxidase inhibitors (MAOIs), selective-serotonin reuptake inhibitors (SSRIs), corticosteroids, chemotherapeutic agents, calcium channel blockers, beta-agonists, and theophylline (Neikrug 2010; Moghadam-Kia 2010; Nerbass 2011; Bercovitch 2012).
Stimulants (e.g., caffeine and nicotine) contribute to insomnia by making it harder for the brain to achieve the state of relaxation needed for sleep. The half-life (amount of time it takes the body to break down 50% of a dose) of caffeine is between three and seven hours; larger amounts and/or repeated doses of caffeine lead to slowed caffeine clearance, causing caffeine's effects to last even longer (Roehrs 2008). As a result, caffeine consumption can impair sleep for many hours. Although, some studies have found that mild caffeine consumption in the morning does not impair sleep (Youngberg 2011).
Nicotine use and nicotine withdrawal can contribute to insomnia (Jaehne 2009). Even those undergoing nicotine replacement therapy (to quit smoking) experience the adverse effects of nicotine on sleep (Mills 2010).
While most people think of alcohol as a sedative, it increases dopamine release within the brain, which has a stimulating effect (Hendler 2013). Chronic alcohol use is associated with insomnia, as is alcohol withdrawal (Brower 2008).
Shift work sleep disorder. Shift work sleep disorder is a type of insomnia in which non-standard work schedules (such as rotating shifts, on-call work, or permanent night shifts) trigger a disconnect between the body's circadian rhythm and time (Kolla 2011).