What is insomnia — the clinical definition
Short answer
Strong evidenceInsomnia is the persistent inability to fall asleep, stay asleep, or return to sleep — occurring at least three nights a week, for at least three months, with daytime consequences. Anything shorter than three months is short-term sleep difficulty, not clinical insomnia.
Key points
- DSM-5 and ICSD-3 criteria: 3 nights/week, 3 months, daytime impact, adequate opportunity.
- Onset insomnia (trouble falling asleep), maintenance insomnia (waking in the night), and mixed are the three patterns. Treatment varies.
- Acute insomnia (under 3 months) is usually triggered by a stressor and resolves. Becoming chronic requires perpetuating factors — often conditioned arousal.
- Insomnia commonly coexists with anxiety, depression, and chronic pain. Treating either side helps the other.
- CBT-I is the first-line treatment per every major guideline (AASM, NICE, European Sleep Research Society). More effective long-term than medication.
The clinical criteria
Both the DSM-5 (the psychiatric reference) and the ICSD-3 (the sleep-medicine reference) define insomnia with five criteria that all must be met:
- A complaint of difficulty initiating sleep, maintaining sleep, or non-restorative sleep.
- Adequate opportunity for sleep (i.e. you allow enough time in bed; you're not just keeping yourself awake).
- The difficulty occurs at least three nights a week.
- The difficulty has persisted at least three months.
- The sleep difficulty causes daytime impact — fatigue, cognitive impairment, mood disturbance, or functional disruption.
Below three months, it's acute insomnia or short-term sleep difficulty. Most acute insomnia resolves on its own once the precipitating stressor lifts.
The three patterns
Clinicians distinguish three insomnia patterns because their likely causes — and best treatments — differ:
- Onset insomnia — difficulty falling asleep at the start of the night. Often driven by cognitive hyperarousal, circadian misalignment, or evening stimulant use.
- Maintenance insomnia — waking during the night and struggling to return. Often driven by alcohol, sleep apnoea, cortisol spikes, or age.
- Mixed insomnia — both. Often the longest- standing, often associated with conditioned arousal where the bed itself has become a trigger for wakefulness.
The 3-P model — how acute becomes chronic
Spielman's 3-P model (1987) explains why some short-term sleep difficulty resolves and some becomes chronic:
- Predisposing factors — genetic propensity to hyperarousal, anxiety traits, family history of insomnia.
- Precipitating factors — the stressor that starts the sleep difficulty (illness, life event, schedule change).
- Perpetuating factors — what keeps it going after the stressor is gone. Almost always behavioural: spending too long in bed, daytime napping, conditioned arousal, anxiety about sleep.
CBT-I targets perpetuating factors directly. That's why it works long after the original trigger has lifted.
Comorbidities
Insomnia commonly coexists with:
- Anxiety and depression — bidirectional. Each worsens the other.
- Chronic pain — pain disrupts sleep; poor sleep worsens pain perception.
- Sleep apnoea — often the underlying mechanism for what gets diagnosed as maintenance insomnia.
- Restless legs syndrome — uncomfortable sensations driving onset difficulty.
Treating either side of a comorbid pair often improves both — but it's essential to identify them. CBT-I works on insomnia comorbid with depression; CPAP works on insomnia driven by apnoea.
First-line treatment
Every major sleep-medicine guideline since 2016 recommends CBT-I as the first-line treatment for chronic insomnia, in preference to medication. The evidence is strong: CBT-I outperforms sleep medication long-term and has no dependency risk.
See CBT-I explained for practical detail, and sleep efficiency calculator to see whether you meet the threshold where CBT-I is most effective.