The Most Common Causes of Insomnia According to Research

Insomnia affects roughly one in three adults at some point in their lives and becomes a chronic condition for about 10% of the population. Despite how common it is, it is also one of the most misunderstood sleep disorders. Many people assume it is simply stress or bad habits. The research tells a more detailed story.

What Insomnia Actually Is

Insomnia is defined clinically as difficulty falling asleep, staying asleep, or waking too early, occurring at least three nights per week for at least three months, causing meaningful distress or functional impairment. Occasional poor nights do not qualify. Insomnia is a persistent pattern.

There is also an important distinction between acute and chronic insomnia. Acute insomnia is short-term, often triggered by a specific stressor or life event, and usually resolves when the stressor passes. Chronic insomnia persists after the original trigger is gone, because the condition has developed its own self-maintaining mechanisms.

The Three-Factor Model

The most widely accepted model of chronic insomnia, developed by Arthur Spielman in the 1980s, identifies three contributing factors: predisposing factors, precipitating factors, and perpetuating factors.

Predisposing factors are biological and psychological traits that make someone vulnerable to insomnia. These include naturally high baseline arousal, a tendency to worry, a hyperreactive stress response, and genetic factors that affect sleep regulation.

Precipitating factors are the events or circumstances that trigger an episode of insomnia. A stressful life event, an illness, shift work, a new baby, grief, or any significant disruption to routine can trigger the first episodes of difficulty sleeping.

Perpetuating factors are the behaviours and thought patterns that maintain insomnia long after the original trigger is gone. Spending too long in bed trying to force sleep, napping to compensate, developing anxiety about sleep itself, and building negative associations with the bedroom all extend and entrench the problem.

The Most Common Causes

Hyperarousal

Research consistently identifies hyperarousal as the most fundamental mechanism in chronic insomnia. People with insomnia show elevated cortisol levels, higher core body temperature, higher metabolic rate, and greater high-frequency brain wave activity at night compared to normal sleepers. The brain is physiologically too activated to transition into sleep.

This hyperarousal has both trait and state components. Some people are constitutionally more aroused as a baseline. Others develop elevated arousal as a result of conditioning: lying in bed awake repeatedly trains the brain to associate the sleep environment with wakefulness and anxiety rather than rest.

Anxiety and Depression

Anxiety disorders and depression are among the strongest predictors of insomnia. The relationship is bidirectional. Anxiety produces hyperarousal that makes sleep difficult. Insomnia worsens anxiety by degrading emotional regulation and increasing cognitive vulnerability to worry. The two conditions feed each other.

Depression disrupts sleep architecture directly, particularly suppressing slow wave sleep and altering REM timing. People with depression often wake early in the morning, unable to return to sleep, and feel profoundly unrefreshed despite spending adequate time in bed.

Poor Sleep Hygiene

While sleep hygiene alone rarely explains chronic insomnia, poor sleep practices are almost always present as maintaining factors. Irregular sleep and wake times disrupt the circadian clock. Spending excessive time in bed erodes sleep pressure. Using the bedroom for activities other than sleep weakens the brain's association between the bed and sleep. Stimulating activity close to bedtime maintains the arousal that prevents sleep onset.

For a detailed look at what not being able to sleep at night looks like in practice and what to do about it, see our article on can't sleep at night.

Conditioned Arousal

This is the perpetuating factor that explains why insomnia persists for months or years after the original cause is long gone. After repeated nights of lying awake in bed, the brain forms a conditioned association: bed equals wakefulness. The sleep environment itself becomes a trigger for arousal rather than relaxation.

This conditioning is powerful and largely unconscious. The person may feel sleepy on the couch but become alert the moment they lie in bed. This is a conditioned response, not a sleep disorder per se, but it is the mechanism that keeps the insomnia going.

Circadian Disruption

Shift work, irregular schedules, travel across time zones, and excessive artificial light at night all disrupt circadian timing and can initiate or maintain insomnia. When the body's internal clock is misaligned with the required sleep schedule, falling asleep at the target time is biologically difficult regardless of habits or stress levels.

Medications and Substances

Many commonly used medications disrupt sleep as a side effect, including some antidepressants, beta blockers, corticosteroids, and decongestants. Alcohol, while sedating initially, disrupts sleep architecture and often causes early morning waking. Caffeine consumed too late in the day suppresses sleep pressure and delays onset.

Medical Conditions

A number of medical conditions directly impair sleep: sleep apnea, restless legs syndrome, chronic pain, acid reflux, thyroid disorders, and menopause-related hormonal changes. Treating the underlying condition is essential in these cases, since sleep-focused interventions alone will not resolve the problem.

What the Research Recommends

Cognitive Behavioural Therapy for Insomnia (CBT-I) is the first-line treatment recommended by the American College of Physicians and most major sleep medicine organisations, ahead of sleep medication. It directly addresses the hyperarousal, conditioned associations, and perpetuating behaviours that maintain chronic insomnia.

For a full breakdown of what CBT-I involves and how effective it is, see our article on CBT for insomnia.

What This Means for Your Sleep

Insomnia is a learned condition as much as a medical one. The original cause may have been stress, illness, or life disruption. What keeps it going is a combination of physiological hyperarousal and conditioned associations that the brain has built up through repeated nights of difficult sleep. This is why addressing only the original stressor is often insufficient. The maintaining factors need to be addressed directly, and CBT-I is the most effective way to do that.