Sleep Science · Part 6 of 6
By Cambridge Sleep Sciences · Medical review: Dr Chris Dickson, 2 July 2026.
There's a threshold beyond which improving sleep hygiene, adjusting your light environment, and cutting evening caffeine simply won't move the needle. For a substantial proportion of the population, poor sleep isn't primarily a habit problem. It's a clinical one. Knowing the difference matters, both because the right interventions are completely different, and because undertreated sleep disorders carry serious long-term health consequences.
This post provides an accessible overview of the most common sleep disorders, their mechanisms, and what effective treatment actually looks like.
Insomnia: more than difficulty sleeping
Insomnia is the most prevalent sleep disorder, affecting approximately 10 to 15% of adults chronically. 1 It's characterised by difficulty falling asleep, staying asleep, or both, on at least three nights per week, for at least three months, despite adequate opportunity for sleep, and it produces meaningful daytime impairment.
It's worth being precise about what insomnia is. It is not simply sleeping less than you'd like. It is a disorder of hyperarousal, a state in which the nervous system is physiologically over-activated relative to the demands of sleep. People with chronic insomnia show elevated metabolic rates, higher baseline cortisol, and increased high-frequency brain activity at sleep onset compared with normal sleepers. In other words, their brains are not quieting down when they attempt to sleep. They're idling too hot.
This is why sedative medications, while effective in the short term, don't address the underlying condition. They suppress the nervous system pharmacologically without correcting the arousal pattern driving the insomnia. When the medication stops, the pattern remains.
The first-line treatment for chronic insomnia is not medication. It is Cognitive Behavioural Therapy for Insomnia, known as CBT-I. Multiple clinical guidelines, including those from the American Academy of Sleep Medicine, recommend CBT-I ahead of pharmacological treatment. The evidence shows it produces equivalent or superior outcomes to sleep medications in the short term, and significantly better outcomes over the long term. 2
CBT-I targets the specific cognitive and behavioural patterns that perpetuate insomnia: dysfunctional beliefs about sleep, compensatory behaviours (excessive time in bed, napping) that weaken sleep drive, and the conditioned arousal that develops when the bed becomes associated with wakefulness. It has also been adapted for digital delivery and shown to be effective via smartphone app, which is relevant given that access to trained CBT-I practitioners is limited.
Insomnia is also strongly bidirectional with depression and anxiety. Disturbed sleep is both a symptom and an independent risk factor for depressive disorder. A meta-analysis found that insomnia is associated with a roughly two-fold increased risk of developing depression. 3 This bidirectionality means that treating insomnia often has secondary benefits for mental health, and that mental health treatment without addressing sleep is frequently incomplete.
Sleep apnoea: the disorder most people don't know they have
Obstructive sleep apnoea (OSA) affects an estimated 2 to 9% of adults, with prevalence rising sharply with age and weight. The majority of cases remain undiagnosed. 4 This gap matters enormously, because untreated OSA is one of the most significant modifiable risk factors for cardiovascular disease, hypertension, stroke, and type 2 diabetes in the population.
In OSA, the upper airway repeatedly collapses during sleep, partially or fully obstructing airflow. Each event, an apnoea, typically lasts 10 to 30 seconds, triggers a micro-arousal that restores muscle tone and reopens the airway, and then repeats. In moderate to severe OSA, this can happen dozens or hundreds of times per night. The person rarely wakes fully and is often unaware of the events, but the sleep architecture is profoundly fragmented, and each apnoea event triggers a sympathetic nervous system activation and a brief hypoxic episode.
The cumulative physiological cost of hundreds of these events per night, sustained over years, is substantial. Chronic OSA creates a state of intermittent hypoxia and sleep fragmentation that independently drives inflammation, endothelial dysfunction, insulin resistance, and elevated blood pressure.
Classic symptoms include loud snoring, witnessed breathing pauses, morning headaches, unrefreshing sleep, and excessive daytime sleepiness. These are absent more often than you'd expect, particularly in women, who more frequently present with insomnia-like symptoms and fatigue.
Diagnosis requires a sleep study (polysomnography or a validated home sleep test). Treatment depends on severity: continuous positive airway pressure (CPAP) therapy is the gold standard for moderate-to-severe OSA and has been shown to reduce blood pressure, improve insulin sensitivity, and decrease cardiovascular risk in treated patients. Mandibular advancement devices (custom oral appliances) are an effective alternative for mild-to-moderate OSA and in patients who can't tolerate CPAP.
Weight loss meaningfully reduces OSA severity in overweight individuals, but is rarely sufficient alone.
Restless legs syndrome: an often-missed cause of sleep disruption
Restless Legs Syndrome (RLS) affects around 5 to 10% of adults and is characterised by an uncomfortable urge to move the legs. It's typically described as a crawling, itching, or aching sensation that occurs predominantly in the evening and at rest, and is temporarily relieved by movement. 5 The result is difficulty falling asleep and maintaining sleep, with significant daytime consequences.
RLS has a neurological basis involving dopaminergic pathways, the same systems implicated in Parkinson's disease, with which RLS shares genetic links. 6 But a significant proportion of RLS cases have a secondary cause that is treatable: iron deficiency. Ferritin levels below approximately 75 µg/L are associated with worsened RLS symptoms, and iron supplementation in deficient individuals can produce substantial improvement. This is clinically underappreciated. Many people with RLS have never had their ferritin tested.
Circadian rhythm disorders: when the clock is out of phase
Circadian rhythm sleep-wake disorders are distinct from insomnia. The problem is not an inability to sleep. It's that the internal clock is shifted significantly out of alignment with conventional social schedules.
In Delayed Sleep Phase Disorder (DSPD), the most common form in younger adults, the circadian clock is set consistently later than the social norm. The person cannot fall asleep until 2 to 4am or later, cannot wake naturally until late morning, and sleeps normally if allowed to follow their endogenous rhythm. The condition is frequently misdiagnosed as insomnia.
Treatment involves targeted bright light therapy in the morning (to shift the clock earlier), strategic melatonin use in the early evening (as a circadian signal rather than a sedative), and chronotherapy. Light therapy is particularly effective and supported by clinical evidence across multiple circadian rhythm disorders. 7
When to seek help
The threshold for professional assessment is lower than most people realise. It's worth consulting a doctor or sleep specialist if:
- You snore loudly, wake with headaches, or feel unrefreshed regardless of how much you sleep. These are red flags for sleep apnoea.
- You've had difficulty sleeping three or more nights a week for three or more months, and it's affecting your daytime function.
- You experience uncomfortable leg sensations at night that disrupt sleep onset.
- You cannot fall asleep until the early hours consistently, despite wanting to sleep earlier.
- You rely on medication or alcohol to sleep regularly.
Sleep disorders are among the most treatable conditions in medicine. They are significantly undertreated because people normalise poor sleep or manage symptoms rather than addressing causes.
Takeaways
Chronic insomnia affects 10 to 15% of adults and is driven by physiological hyperarousal, not simply bad habits. CBT-I is the evidence-supported first-line treatment, not medication. Sleep apnoea is highly prevalent, largely undiagnosed, and carries serious cardiovascular and metabolic consequences when untreated. CPAP therapy is highly effective. Restless Legs Syndrome is frequently iron-deficiency-related. Ferritin testing is a simple, important first step. Delayed sleep phase disorder is a circadian condition, not insomnia. It responds to light therapy and melatonin, not standard sleep hygiene. The bar for seeking professional assessment should be low. These conditions are common, consequential, and treatable.
This series has covered the foundations of sleep science, from the biology of a single night to the disorders that disrupt it. If anything above sounds familiar, talk to a doctor first. For the work Cambridge Sleep Sciences™ does on sleep technology, see how SleepEngine™ works.
Explore the sleep science series
References
- Morin CM, Benca R. Chronic insomnia. Lancet. 2012;379(9821):1129-41.
- Walker J, et al. Cognitive Behavioral Therapy for Insomnia (CBT-I): a primer. Klin Spec Psihol. 2022;11(2):123-137.
- Baglioni C, et al. Insomnia as a predictor of depression: a meta-analytic evaluation. J Affect Disord. 2011;135(1-3):10-9.
- Peppard PE, et al. Increased prevalence of sleep-disordered breathing in adults. Am J Epidemiol. 2013;177(9):1006-14.
- Allen RP, et al. Restless legs syndrome/Willis-Ekbom disease diagnostic criteria. Sleep Med. 2014;15(8):860-73.
- Peeraully T, Tan EK. Linking restless legs syndrome with Parkinson's disease. Transl Neurodegener. 2012;1(1):6.
- Abbott SM, et al. Circadian rhythm sleep-wake disorders. Psychiatr Clin North Am. 2015;38(4):805-23.
