What is Insomnia?
Almost everyone has a bad night's sleep now and then. But insomnia is something more specific — and understanding what it actually is turns out to matter a lot, because it changes what kind of help is likely to work.
The clinical definition
In Cognitive Behavioral Treatment of Insomnia: A Session-by-Session Guide, sleep researcher Michael Perlis and colleagues open by pinning down exactly what counts as insomnia, since the word is used loosely in everyday speech. Clinically, insomnia disorder is a complaint, not just a symptom: a person is dissatisfied with the quantity or quality of their sleep, and has trouble falling asleep, staying asleep, waking too early, or getting sleep that doesn't feel refreshing — even though they had a reasonable opportunity to sleep. To be diagnosed as a disorder, this needs to happen at least three nights a week for at least three months, and cause real daytime distress or impairment (fatigue, poor concentration, low mood, reduced performance at work or school). These are the same core criteria used in the two major diagnostic systems clinicians rely on, the DSM-5 and the International Classification of Sleep Disorders, Third Edition (ICSD-3).
Two things stand out in this definition. First, insomnia is about the daytime consequences as much as the night-time struggle — poor sleep that doesn't bother you isn't classified as insomnia. Second, it has to occur despite adequate opportunity to sleep; someone who simply doesn't allow themselves enough time in bed doesn't meet the definition, however tired they feel.
Where the word comes from
"Insomnia" traces back to Latin: in- ("not") plus somnus ("sleep"), giving insomnis, "sleepless." The English word entered use in the early-to-mid 1600s, with the modern, re-Latinised spelling settling in by the mid-1700s. It's a linguistic cousin of words like "somnolent" and, more distantly, "hypnosis," which comes from the Greek equivalent, hypnos.
How common is it in Australia?
Sleep problems are strikingly common. National surveys from the Sleep Health Foundation have found that roughly two in three Australian adults report at least one sleep problem, and about 20% report symptoms consistent with significant insomnia. When researchers applied the stricter, formal ICSD-3 criteria to a large national sample, the prevalence of diagnosable chronic insomnia disorder came out at around 15%, with women reporting it somewhat more often than men.
What else can cause poor or non-restorative sleep?
Insomnia is only one cause of a bad night. Others include:
Obstructive sleep apnoea (OSA) — breathing repeatedly stops and starts during sleep, fragmenting it.
Restless legs syndrome and periodic limb movements — uncomfortable urges to move the legs, or involuntary jerking during sleep.
Circadian rhythm disorders — the body's internal clock is out of sync with the desired sleep schedule (common in shift workers).
Medical and mental health conditions — chronic pain, depression, anxiety, and menopause-related symptoms can all disrupt sleep.
Substances — caffeine, alcohol, nicotine, and some medications.
Poor sleep habits or environment — irregular schedules, screens in bed, an uncomfortable bedroom.
How common are these other sleep issues?
They're widespread too. Australian survey data suggest around 8–9% of adults have diagnosed sleep apnoea (with many more undiagnosed), and about 18% report restless legs symptoms. Combined, doctor-diagnosed sleep disorders affect roughly one in five Australian adults.
What should you do if you think you have insomnia?
Track your sleep for one to two weeks in a simple diary. Log your bedtime, roughly how long it took to fall asleep, any awakenings, and your final wake time. This turns a vague sense of "I sleep badly" into concrete figures: how long you actually take to fall asleep, how much time you spend awake overnight, and your sleep efficiency (time asleep ÷ time in bed). That's the exact information a GP or sleep clinician needs to confirm insomnia rather than another sleep disorder.
See your GP — they can rule out other sleep disorders and medical causes.
Ask about Cognitive Behavioural Therapy for Insomnia (CBT-I), the first-line, evidence-based treatment, rather than jumping straight to sleeping pills.
If OSA or restless legs are suspected, ask about referral to a sleep specialist or sleep study.
References
Perlis, M. L., Jungquist, C., Smith, M. T., & Posner, D. (2005). Cognitive Behavioral Treatment of Insomnia: A Session-by-Session Guide. Springer.
American Psychiatric Association. (2013). Diagnostic and Statistical Manual of Mental Disorders (5th ed.).
American Academy of Sleep Medicine. (2014). International Classification of Sleep Disorders (3rd ed.).
Adams, R. J., Appleton, S. L., Taylor, A. W., Gill, T. K., Lang, C., McEvoy, R. D., & Antic, N. A. (2017). Sleep health of Australian adults in 2016: Results of the 2016 Sleep Health Foundation national survey. Sleep Health, 3(1), 35–42.
Reynolds, A. C., Appleton, S. L., Gill, T. K., Melaku, Y. A., & Adams, R. J. (2022). Insomnia prevalence varies with symptom criteria used with implications for epidemiological studies. Nature and Science of Sleep, 14, 775–787.
Reynolds, A., Appleton, S. L., Gill, T. K., & Adams, R. (2019). Chronic Insomnia Disorder in Australia: A Report to the Sleep Health Foundation. Flinders University / Sleep Health Foundation.
Harper, D. Online Etymology Dictionary — entry for "insomnia."