Insomnia is what you have when sleep will not start, will not hold, or ends too early even though the night had room for it, and the day after pays for it. Clinicians sort it by how long it has run: short-term insomnia in days or weeks, chronic insomnia in months. That line decides what helps.

What counts as insomnia, and what does not

The definition has four parts, and the one people skip is the third. Insomnia disorder, under the DSM-5 criteria applied in a 2025 meta-analysis of general-population studies, means difficulty starting or staying asleep, or unwanted early waking, on at least 3 days a week for at least 3 months, despite an adequate chance to sleep, and with real distress or impairment during the day.¹

Start with that third part, the adequate chance. A nurse who clocks off at midnight and is up again at five is short of sleep, and so is the parent of a newborn; neither has insomnia, because the room for sleep was never there.

The fourth part, the daytime cost, is what turns a bad night into a disorder. The US National Heart, Lung, and Blood Institute (NHLBI) says insomnia can interfere with daily activities, leave you sleepy during the day, and affect memory and concentration.² Ragged nights with fine days do not meet the definition.

So the test is not hours logged, but whether the night had room for sleep and what the next day cost you. It is also why the everyday habits that help most people sleep better are a different subject from treating insomnia: habits widen the opportunity, and opportunity is not the part that is broken.

Short-term insomnia and chronic insomnia are different problems

Short-term insomnia is a reaction; chronic insomnia has outlived its cause. The NHLBI says short-term insomnia may follow stress or a change in your schedule or environment and can last a few days or weeks, while chronic insomnia happens 3 or more nights a week, runs longer than 3 months, and cannot be fully explained by another health problem. Either type can be trouble falling asleep, trouble staying asleep or poor-quality sleep.² The UK’s NHS draws the same 3-month line, calling them short-term and long-term insomnia.³

Myth

Insomnia just means you are not getting enough sleep.

Fact

It means sleep will not come when the chance is there. The US NHLBI keeps the chronic label for trouble on 3 or more nights a week for longer than 3 months that another health problem does not fully explain.

Why the split matters: a bad couple of weeks after a bereavement, a house move or a block of night shifts is an ordinary response to an event, and it usually fades as the event does. Once it has run past three months, something else is holding the problem in place, and it is rarely the original trigger. So the useful thing to take to an appointment is the calendar: when this started, and how many nights a week since.

How common insomnia really is, and why the estimates disagree

Insomnia is common without being universal, and the number you meet depends on how the question was asked.

The study

Moderate evidence

One adult in eight, where a clinician made the diagnosis: 14 of 47 pooled studies

Researchers led by Annemieke van Straten at Vrije Universiteit Amsterdam pooled 47 studies that measured insomnia disorder in random samples of the general population, by diagnostic interview, by DSM-based self-report questions, or with a questionnaire whose cut-off was set against the DSM criteria. In the 14 studies where an interviewer or clinician made the diagnosis, the method the authors call the gold standard, about 12 percent met the criteria. Self-report versions of the same criteria gave a higher figure, looser questionnaire cut-offs higher still, and the better-quality studies reported lower rates than the weaker ones.¹

What that is worth to you: roughly one adult in eight, on the strictest measure, is not a rounding error, and it is also not everyone. The looser the method, the larger the number, so a page claiming a third of adults have insomnia is almost certainly quoting a questionnaire cut-off rather than a diagnosis. So is your own score on a sleep quiz: a reason to look closer, not a diagnosis.

What causes insomnia, and what keeps it going

Most cases start with something recognizable. The NHS names stress, anxiety or depression, noise, an uncomfortable bed or a room that is too hot or cold, alcohol, caffeine or nicotine, jet lag and shift work as the most common causes, and adds conditions that disturb sleep from the inside, from restless legs and an overactive thyroid to long-term pain, menopause and sleep apnea.³ The NHLBI adds the ordinary risks around them: older age, a family history, an irregular routine, long daytime naps, screens close to bedtime, and worrying about whether you will sleep.⁴

That last one is the hinge: starting a problem and keeping it going are different jobs.

1. Sleep opportunity: the hours you hand to sleep (an earlier bedtime, a later lie-in, an afternoon nap) after a run of bad nights
2. Sleep ability: how much sleep your body can actually produce tonight, which the extra hours do not change
3. The gap: the leftover stretch has to be filled with wakefulness, and in the 3P model that is what keeps insomnia going

Researchers separate the two, and the standard account is the 3P model, published by Arthur Spielman and colleagues in 1987 and described by the sleep researcher Michael Perlis and colleagues as the first model of insomnia to win wide acceptance. It has three ingredients: predisposing factors you carry, such as a tendency to worry or a partner on an incompatible schedule; a precipitating event, usually life stress or illness; and perpetuating factors, the things a person does to cope.⁵

The perpetuating factors are what treatment targets, because they are the only ones still active months later. Perlis and colleagues name three: doing non-sleep things in the bedroom, staying in bed while awake, and spending more time in bed overall. The last is the model’s main concern, and the one that looks like ordinary good sense. In the short term it even seems to work, because some lost sleep is recovered; over months it opens a mismatch between sleep opportunity and sleep ability, and the wider the gap, the more of the night is spent lying awake, whatever started the problem.⁵

Put a clock on it. After a rough week, someone who used to sleep from eleven to half past six starts going up at ten and lying in until half past seven. The body makes no more sleep than before, so the extra hour and a half is not extra sleep: it is extra waiting, in the place they most need to associate with sleeping.

1. 1A run of bad nightsafter stress, illness or a schedule change
2. 2You give sleep more roomearlier bedtime, later lie-in, an afternoon nap
3. 3Time in bed exceeds sleep abilitythe gap fills with wakefulness
4. 4More hours awake in bedreading, worrying, clock-watching
5. 5The bed stops meaning sleepold sleep cues now cue being awake

Then repeat from “A run of bad nights”

That last step has a name. Perlis and colleagues describe how the cues for sleep, the bed, the bedroom, the hour, get paired with everything except sleeping until they stop bringing it on: stimulus dyscontrol.⁵ It is why someone can be heavy-eyed on the sofa and wide awake the moment the bedroom light goes off.

Why guidelines put therapy ahead of pills

Named US guidelines agree on the order: therapy before pills. The American Academy of Sleep Medicine’s 2021 US guideline, which says treatment should follow a diagnosis made under ICSD-3 or DSM-5 criteria, makes a single strong recommendation, for multicomponent cognitive behavioral therapy for insomnia, known as CBT-I. Its components, including stimulus control, sleep restriction and relaxation, get only conditional recommendations on their own, and it suggests that clinicians not use sleep hygiene as a single-component therapy, a conditional recommendation against the advice most people have already tried.⁶

The American College of Physicians reached the same place five years earlier: its 2016 US guideline recommends CBT-I as the initial treatment for all adults with chronic insomnia disorder, a strong recommendation on moderate-quality evidence, and puts medication second, on low-quality evidence, as a shared decision with a clinician about short-term use when CBT-I alone has not worked.⁷

In the UK, the NHS says a GP will look for the cause of your insomnia, may offer cognitive behavioral therapy or refer you to a sleep clinic, and notes that GPs now rarely prescribe sleeping pills, which can have serious side effects and which you can become dependent on.³

The over-the-counter aisle is where the evidence thins. The AASM’s 2017 US guideline on medicines suggested that clinicians not use melatonin, diphenhydramine, valerian or tryptophan for chronic insomnia, all weak recommendations, on evidence the task force said it had downgraded partly because of who funds most drug trials and the publication bias that follows.⁸

Regulation then differs by country. UK rules make melatonin prescription-only, used mainly for short-term sleep problems from age 55 upwards.⁹ The US sells it as a dietary supplement, a category the FDA says it has no authority to approve for safety and effectiveness before it reaches the public.¹⁰ The NIH’s National Center for Complementary and Integrative Health, reading the AASM and ACP guidelines above, says there is not enough strong evidence on melatonin for chronic insomnia to recommend its use, that information on the long-term safety of supplementing with it is lacking, and that people with epilepsy and people taking blood thinners need to be under medical supervision when they take it. It adds that parents considering melatonin for a child should speak with a health care provider first.¹¹ Ask a pharmacist or your doctor before you take any sleep aid, prescription or not, and especially in pregnancy or breastfeeding, alongside other medicines, or with a health condition.

Getting help: what is urgent tonight, and what needs a doctor soon

Sleepiness at the wheel and thoughts of self-harm need attention today; snoring with pauses in breathing, and months of broken nights, need a doctor within weeks; the rest belongs at your next appointment. The sorting is ours; each piece of advice is attributed to the US or UK body that published it.

• Now, today. Do not drive while you are fighting to stay awake. The US National Institute for Occupational Safety and Health lists the warning signs: heavy eyelids or frequent blinking, wandering thoughts, no memory of the last few miles, missing exits, drifting out of your lane or hitting a rumble strip.¹² Its advice for a driver who feels drowsy is to pull off at the next exit or rest area and stop driving, and it notes that a nap of even 15 to 20 minutes can help.¹³ If thoughts of suicide or self-harm come alongside the sleeplessness, treat that as the urgent problem rather than the sleep. In the US, the National Institute of Mental Health points to the 988 Suicide and Crisis Lifeline, reachable by phone, by text or by chat at 988lifeline.org, and says to call 911 when life is in danger.¹⁴ In the UK, the NHS says to call 999 or go to A&E if someone’s life is at risk or you do not feel able to keep yourself or another person safe, that Samaritans listen on 116 123, and that for urgent help short of an emergency you can use 111 online or call 111 and select the mental health option.¹⁵ For urgent medical help elsewhere in the UK, the NHS’s own 111 page points to NHS 24 111 in Scotland, NHS 111 Wales, and nidirect in Northern Ireland.¹⁶ In any other country, call your local emergency number.
• Soon, within weeks. Loud snoring, gasping, snorting or choking noises, and breathing that stops and starts in sleep are the signs of sleep apnea the NHS names, and it says the condition can be serious if it is not diagnosed and treated; in the UK, you must not drive once sleep apnea with excessive sleepiness has been confirmed, until symptoms such as feeling very tired are under control.¹⁷ The NHS also points to three reasons to see a GP about insomnia: changing your sleeping habits has not helped, the trouble has run for months, or it is affecting your daily life in a way that makes it hard to cope.³
• Routine, at your next visit. Raise persistent poor sleep even when it feels manageable, and bring evidence. The NHLBI says it may help to keep a sleep diary for 1 to 2 weeks before the appointment, and that a provider will ask about your symptoms, risk factors, health history and family history before considering blood tests, a wrist activity monitor or a sleep study.¹⁸

WiserHours

Worth writing down before you go

• A sleep diary covering 1 to 2 weeks
• When the trouble started, and what was happening at the time
• How many nights a week, and how many months so far
• What the next day costs you: sleepiness, focus, mood
• Medicines and supplements you take, plus caffeine, alcohol and nicotine
• Whether anyone has heard you snore loudly, gasp or stop breathing

Find this checklist online:
https://wiserhours.com/sleep/insomnia-types-and-causes/#checklist-insomnia-types-and-causes-appointment

The bottom line

Two questions sort out most of this: did the night have room for sleep, and how long has it been this way? If the answer is 3 or more nights a week for longer than 3 months, the trigger has probably stopped mattering and the habits built around it have taken over. That is a conversation for a doctor, and the American College of Physicians’ 2016 US guideline is clear on the order: therapy first, medication only as a considered second step.⁷