Why can't I sleep? What causes insomnia, and the tests that check the medical causes

Most of the time you can't sleep because of something in the day or the bedroom: stress and worry, caffeine, alcohol or nicotine, screens late at night, an irregular schedule or shift work, or a room that is noisy, bright, hot or cold. ²⁵ When it keeps happening 3 or more nights a week for more than 3 months, it is called chronic insomnia, and the first treatment US guidelines recommend is cognitive behavioral therapy for insomnia (CBT-I), a short program that retrains sleep habits and the worry around them. ¹⁶⁷ Medical causes, such as depression, menopause, sleep apnea (breathing that stops and starts in sleep), restless legs, an overactive thyroid and diabetes, are worth checking when the everyday causes don't explain it. ⁵⁸¹¹¹²

What counts as insomnia, and how much sleep is normal?

Insomnia means trouble falling asleep, staying asleep or getting good-quality sleep even though you have the time and a decent place to sleep. ¹ The NHS lists the signs: finding it hard to go to sleep, waking several times in the night, lying awake, waking early and not getting back to sleep, and feeling tired, irritable and unable to concentrate the next day. ⁵ On average adults need 7 to 9 hours, and the NHS puts the test plainly: "You probably do not get enough sleep if you're constantly tired during the day." ⁵

Length is what separates a rough patch from a problem. Short-term insomnia follows stress or a change in schedule or environment and lasts days or weeks. ¹ The NHLBI defines chronic insomnia as sleeplessness that "occurs 3 or more nights a week, lasts more than 3 months, and cannot be fully explained by another health problem." ¹ By then the original trigger has often faded, and what keeps it going is worry about sleep, clock-watching and habits built around bad nights. ² That is the one distinction to hold on to: chronic insomnia is treated with CBT-I, a structured program, while the everyday causes are fixed by changing the day.

What to do when you can't sleep tonight

The most searched question about sleep is the simplest one: what to do at 2am when you are wide awake. The advice from CBT-I is counterintuitive. Lying in bed trying harder tends to teach the brain that bed is a place for worrying, so the stimulus control part of CBT-I helps you "link being in bed with being asleep." ⁴ In practice:

1. 1Stop checking the clockWatching the clock and worrying about how much sleep you are losing feed insomnia. ² Turn the clock away.
2. 2Get up if you can't settleIf you can't sleep, get out of bed and do something calm in low light, such as reading on paper, until you feel sleepy. The NHLBI describes this as "getting out of bed if you cannot sleep." ⁴
3. 3Keep screens awayPhones, tablets and TV close to bedtime make you more alert. ²⁵
4. 4Go back to bed only when sleepyThe NHS advice is to go to bed only when you are sleepy. ⁵
5. 5Get up at the usual time anywayDon't sleep in after a bad night, and skip the daytime nap; the NHS says to stick to your regular sleeping hours instead. ⁵

Why do I wake up at 3am?

Waking in the night and waking early without getting back to sleep are both standard insomnia symptoms, not a separate condition. ⁵ The usual reasons are the same everyday ones: stress, alcohol in the evening, a warm or noisy room, a full bladder or pain. ²⁵ A few medical patterns are worth knowing. Menopause brings hot flashes and night sweats that wake people. ¹⁰ Sleep apnea causes repeated waking, often with a need to urinate. ⁸ And early waking with low mood and loss of interest is one of the listed signs of depression, which the NIMH describes as "waking too early in the morning." ⁹

Tired but can't sleep: why both happen at once

Feeling exhausted and being able to fall asleep are controlled by different things. A worn-out body can sit alongside an alert mind, which is why stress, worry about sleep and caffeine taken earlier in the day can keep you awake when you feel drained. ² If you are tired every day even after reasonable nights, the cause may not be the sleep at all: fatigue covers low iron, thyroid problems and the other causes, and brain fog covers trouble thinking clearly.

How CBT-I works, and why it comes before pills

The American College of Physicians states: "ACP recommends that all adult patients receive cognitive behavioral therapy for insomnia (CBT-I) as the initial treatment for chronic insomnia disorder." ⁶ The American Academy of Sleep Medicine gives multicomponent CBT-I its only strong recommendation, and advises against using sleep hygiene tips alone as the treatment. ⁷ The NHLBI describes CBT-I as a 6 to 8 week plan that a doctor, nurse or therapist can deliver in person, by phone or online. ⁴ It has five parts:

Sleeping pills exist, and the ACP guideline leaves room for a short course, chosen with a clinician, when CBT-I alone hasn't worked. ⁶ The NHLBI notes side effects such as dizziness and, rarely, doing things while asleep such as walking or driving. ⁴ Over-the-counter sleep aids that contain an antihistamine may make you sleepy, but the NHLBI advises talking to your doctor first because antihistamines can be unsafe for some people, and they can worsen restless legs. ⁴ Melatonin is widely used; NCCIH says it may help with shift work and jet lag, while two clinical guidelines advised against it for chronic insomnia. ¹³

What blood tests can and can't tell you

No blood test diagnoses insomnia. A clinician diagnoses it from your story and a sleep diary, then looks for anything that might be driving it: the NHLBI says a sleep study checks for sleep apnea and other sleep disorders, a wrist sensor called actigraphy tracks rest and activity for 3 to 14 days, and "Blood tests check for thyroid problems or other medical conditions that can affect sleep." ³ So a normal blood panel does not mean nothing is wrong with your sleep; it means the thyroid, iron and blood-sugar causes are less likely.

Getting a useful appointment

• Keep a sleep diary for 1 to 2 weeks: when you go to bed, fall asleep, wake and nap, how sleepy you feel in the day, and when you have caffeine, alcohol or exercise. ³
• Ask a bed partner whether you snore loudly, stop breathing or gasp. ³⁸
• List every medicine and supplement, including cold, allergy and over-the-counter sleep products, with start dates. ³⁴
• Mention high-dose biotin before a thyroid test, since it can make TSH look falsely low; agree any pause before the test with your clinician, and see the thyroid panel guide.
• Mention low mood, anxiety, pregnancy or changing periods; each changes what the clinician looks for. ³
• One blood result is a snapshot; see why trends beat single readings.

Questions to ask your clinician

• Does my pattern fit chronic insomnia, and can you refer me to CBT-I, in person or online?
• Could any of my medicines be keeping me awake?
• Should I have a sleep study for snoring or sleep apnea?
• Is a TSH, ferritin or HbA1c worth checking for me?
• Could perimenopause, low mood or anxiety be part of this?

This page is general education, not medical advice or a diagnosis. Sleep problems usually have more than one cause at once, and the clinician who knows your history is the one to read your results and decide what comes first.

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