Insomnia is usually treated as a single clinical condition with one explanation: stress. Research tells a more complicated story. Persistent problems falling or staying asleep almost always have an identifiable driver, and stress is just one of several. This is part of the reason why the blanket advice to "just relax" rarely solves the problem on its own.
Chronic stress and a nervous system in the wrong gear
Insomnia is indeed most often driven by a stress response that cannot be switched off. The sympathetic nervous system—the body's fight-or-flight mode—is directly incompatible with the parasympathetic state required for falling asleep. Elevated cortisol in the evening, whether from acute stress or chronic overload, keeps the body in a state of high alert exactly when it needs to be winding down. Sleep research consistently finds elevated evening cortisol and measurably increased physiological tension—visible in heart rate variability—in people with chronic insomnia compared to good sleepers, even on nights when the individual feels calm.
An inner clock that has fallen out of sync
The internal clock that determines when the body becomes tired requires constant signals, especially light, to stay precise. Irregular wake-up times, late-evening screen time that suppresses melatonin secretion, and insufficient morning light can shift the circadian clock so far away from the desired sleep rhythm that a pattern emerges which looks like insomnia but is, in truth, a timing problem. This is especially common in people whose daily routines fluctuate significantly and whose bodies never receive a consistent signal to anchor a stable sleep window.
Anxiety and the carousel of thoughts at the wrong moment
Cognitive hyperarousal—the specific pattern of racing thoughts, mentally replaying the day's problems, and the fear of being unable to sleep—is one of the best-documented drivers of chronic insomnia, and it functions mechanistically differently from general stress. Imaging studies show that people with this pattern have increased activity in brain regions associated with self-referential thought and threat monitoring precisely when they are trying to fall asleep. The pattern is self-reinforcing, as the worry about insomnia itself becomes another thought that keeps the brain awake.
Hormonal shifts that directly interfere with sleep architecture
For women, the risk of insomnia changes measurably across the menstrual cycle and during major hormonal transitions. Progesterone has a genuine sleep-promoting effect, and its levels drop sharply in the days before menstruation, a documented factor in premenstrual sleep disturbances. Perimenopause, which can begin as early as one’s mid-30s, brings increasingly unpredictable fluctuations in estrogen and progesterone and is consistently linked to a significant rise in insomnia symptoms, regardless of stress or lifestyle. Women are significantly more likely to be diagnosed with insomnia than men throughout their lifespan, and hormonal variability is one of the primary reasons.
Caffeine, alcohol, and timing that most people underestimate
The five- to six-hour half-life of caffeine means that an afternoon coffee is still substantially active at bedtime, blocking the adenosine signal that builds up sleep pressure throughout the day. Alcohol, which seemingly makes it easier to fall asleep, is one of the most disruptive substances for sleep continuity: it consistently reduces REM sleep and increases awakenings in the second half of the night as it is metabolized. Both are common, underestimated factors that someone might consume daily without ever connecting them to the insomnia they are trying to solve.
An airway that isn't quite open
A less discussed but genuinely significant factor is restricted nasal airflow during sleep, which does not have to be severe enough to cause loud snoring to disrupt sleep continuity. Mild airway constriction lowers nighttime oxygen levels and triggers brief, unremembered arousals throughout the night. This creates a pattern that looks and feels exactly like insomnia—frequent waking, unrefreshing sleep, and difficulty staying asleep—while the actual driver is mechanical rather than psychological.
Why finding the true driver matters
Insomnia is a symptom, not a single diagnosis, and the research is clear: the effectiveness of an intervention depends entirely on which mechanism is truly behind it. Cognitive behavioral therapy for insomnia (CBT-I), the best-supported non-medication treatment, works specifically on the learned associations and cognitive hyperarousal behind stress-driven insomnia, and it is significantly less effective for someone whose actual driver is a circadian timing problem or an undiagnosed airway issue. Someone lying awake at 2 a.m. rarely has only one cause. Identifying which of these mechanisms is yours is a more useful first step than any generic sleep hygiene tip.