If you’ve been lying awake night after night, you’ve probably already noticed the obvious effects: the fog behind your eyes, the irritability, the third coffee by 11am. What’s less obvious — and what most exhausted adults never hear from their GP — is that insomnia doesn’t just drain your energy. Over time, it can quietly push your blood sugar in the wrong direction.
This isn’t a scare story. It’s a well-documented, biologically plausible link that UK researchers have studied directly, and it’s one more reason why treating insomnia properly, rather than just “pushing through” it, matters for your long-term health. Here’s what the evidence actually shows, how the mechanism works, and — most importantly — what you can do about it.
The UK research behind the link
One of the clearest pieces of evidence comes from a 2022 study led by the University of Bristol and funded by Diabetes UK, published in the journal Diabetes Care. The researchers used UK Biobank data from more than 336,000 adults and applied a method called Mendelian randomisation, which helps establish whether one factor is likely to be causing another, rather than the two simply occurring together.
They looked at five sleep traits: insomnia symptoms, sleep duration, daytime sleepiness, napping, and chronotype (whether someone is naturally more of a morning or evening person). Of these five, only frequent insomnia symptoms showed a clear link to higher HbA1c, the blood marker used to track average blood sugar levels over the preceding two to three months.
The researchers estimated that around 27,300 UK adults aged 40 to 70 with frequent insomnia symptoms could potentially benefit, in terms of blood sugar control, if their insomnia were properly treated. That’s not a trivial number, and it’s a strong argument for taking persistent sleeplessness seriously rather than normalising it.
Why poor sleep affects blood sugar: the mechanism
You don’t need to be a biochemist to understand the basics, and understanding what’s actually happening in your body can make the problem feel less abstract. A few things tend to happen when sleep is chronically disrupted:
- Cortisol rises. Poor sleep is a physical stressor, and your body responds by producing more cortisol, a hormone that raises blood glucose and can blunt the effect of insulin.
- Insulin sensitivity drops. Earlier research in people with diabetes found that poor sleepers had notably higher morning blood glucose and higher insulin levels than good sleepers with the same condition, suggesting their bodies were working harder to manage the same amount of sugar.
- Appetite hormones shift. Sleep loss tends to increase ghrelin (which drives hunger) and reduce leptin (which signals fullness), which can lead to more snacking, more sugar cravings, and less motivation to exercise — all of which affect blood sugar indirectly.
- Daytime fatigue reduces activity. When you’re exhausted, you move less. Lower activity levels are independently linked to poorer blood sugar regulation.
None of this means a few rough nights will tip you into diabetes. The concern is specifically with chronic, ongoing insomnia — the kind that lasts months, not the odd bad night before a big meeting.
How to tell if this applies to you
This research is about insomnia as a diagnosable, persistent pattern — not the occasional night of poor sleep everyone has. Clinically, insomnia usually means:
- Difficulty falling asleep, staying asleep, or waking too early, happening at least three nights a week
- This pattern lasting three months or more
- Noticeable daytime impact — fatigue, poor concentration, low mood, or irritability
If that sounds familiar, it’s worth reading more about insomnia symptoms to see how closely your experience matches, rather than assuming it’s “just how you are now.”
The good news: this is treatable, and the benefits go beyond sleep
Here’s the part that should feel encouraging rather than alarming. Insomnia is one of the most treatable sleep conditions there is, and the recommended first-line approach isn’t a pill — it’s Cognitive Behavioural Therapy for Insomnia, or CBT-I.
The online insomnia treatment we offer at The Online Sleep Clinic is built around CBT-I, which works by addressing the behaviours and thought patterns that keep insomnia going, rather than just masking the symptoms. It’s recommended by the UK’s National Institute for Health and Care Excellence (NICE) as a first-line approach, precisely because it produces durable improvements without the dependency risks associated with sleeping pills.
Importantly, because CBT-I improves sleep quality rather than simply sedating you, the downstream effects — on cortisol regulation, appetite hormones, and daytime activity levels — tend to follow naturally. You’re not just sleeping more; you’re giving your body’s metabolic systems a chance to reset. If you want to understand the evidence base in more depth, our page on why CBT-I works goes into the research behind it.
What you can start doing tonight
While working with a specialist produces the most reliable, lasting results, there are a few evidence-based adjustments that can help in the meantime:
- Keep a consistent wake time. Even on weekends. This is one of the strongest anchors for your body clock and, over time, for your metabolic rhythms too.
- Get daylight exposure in the morning. Light exposure helps regulate cortisol’s natural daily rhythm, which in turn supports healthier blood sugar patterns.
- Avoid long “catch-up” lie-ins. They disrupt your sleep drive for the following night and can worsen the insomnia cycle rather than resolve it.
- Don’t fight the clock at 3am. If you’re lying awake for long periods, lying there willing yourself to sleep usually backfires. Our guide on waking up during the night explains what to do instead.
- Be cautious with “just one more coffee.” Caffeine late in the day doesn’t just delay sleep onset; by worsening sleep, it can indirectly add to the same metabolic strain we’ve been discussing.
These steps help, but they’re supportive measures, not a substitute for proper treatment. If insomnia has been going on for months, self-help alone rarely resolves it. For practical, structured strategies, see our article on how to fall asleep fast, and for a fuller picture of non-drug options, our guide to insomnia treatment without medication is a good next step.
Speak to your GP if you’re concerned about diabetes risk
If you have other risk factors for type 2 diabetes — a family history, excess weight around the middle, or you’ve had borderline blood sugar readings before — it’s worth mentioning your sleep difficulties at your next GP appointment. Your GP can check your HbA1c if appropriate and advise on monitoring. This article isn’t a substitute for medical advice, and if you’re currently taking any medication for diabetes or sleep, please don’t change your regimen without speaking to your GP or prescriber first.
Where to go from here
Insomnia is often treated as a nuisance to endure rather than a condition to treat — something you’re expected to just get on with. The research on blood sugar is one more reminder that it deserves more attention than that. For a broader look at how untreated insomnia affects the body over time, our complete guide to insomnia is a useful starting point.
Frequently asked questions
Does insomnia definitely cause type 2 diabetes?
No single study proves insomnia alone causes type 2 diabetes, but UK research using genetic data from over 336,000 adults found that frequent insomnia symptoms were linked to higher blood sugar (HbA1c) in a way that suggested a causal, not just coincidental, relationship. Insomnia is best understood as one contributing risk factor among several, including diet, activity levels, and family history.
Can improving my sleep actually lower my blood sugar?
Improving chronic insomnia can support healthier blood sugar regulation by reducing cortisol output, improving insulin sensitivity, and stabilising appetite hormones. Researchers have specifically suggested that treating insomnia could be a useful tool for preventing or managing type 2 diabetes risk, particularly in people who have frequent sleep difficulties.
How much insomnia is “enough” to worry about this link?
The research focused on frequent, ongoing insomnia symptoms, generally meaning difficulty falling or staying asleep most nights for three months or longer, rather than occasional poor sleep. A single stressful week of bad sleep is not the same as chronic insomnia and is unlikely to meaningfully affect blood sugar on its own.
What’s the most effective treatment for insomnia if I’m worried about my metabolic health?
Cognitive Behavioural Therapy for Insomnia (CBT-I) is recommended by NICE as the first-line insomnia treatment, ahead of sleeping pills, because it addresses the underlying causes of poor sleep rather than just sedating you. Because it improves genuine sleep quality, its benefits for cortisol, appetite regulation and energy levels tend to be more durable than those from medication alone.
If persistent insomnia has been part of your life for a while, it’s worth finding out exactly where you stand. Take our free insomnia self-assessment to get a clear, personalised picture of your sleep difficulties and the right next step — whether that’s self-help strategies or structured CBT-I support. You can also get in touch if you’d like to discuss treatment options directly.