20.11 — Sleep and Its Disorders

The brain has no lymphatic vessels, so for a long time nobody knew how it cleared its waste.

Then it turned out that during sleep, the space between brain cells expands by around 60 percent, and cerebrospinal fluid is pumped through the tissue, washing out metabolic waste — including amyloid-beta, the protein that accumulates in Alzheimer's disease (Chapter 20.4).

This system runs far more actively during sleep than during waking.

Which is the clearest answer yet to why sleep is not optional. It is not rest. It is maintenance that cannot be done while the machine is running.

What happens when you sleep

Two kinds of sleep, alternating in cycles of roughly 90 minutes.

Non-REM sleep, in stages of increasing depth.

Stage 3 is deep, slow-wave sleep — the hardest to wake from, and the stage that produces the feeling of having slept properly. Growth hormone is released, tissue repair happens, and the immune system consolidates.

And it is concentrated in the first half of the night, which is why the first few hours are the ones you cannot skip.

REM sleep — rapid eye movement, vivid dreaming, and a brain almost as active as when awake.

The body is paralysed during REM, except for the eyes and the diaphragm, which stops you acting out dreams. When that paralysis fails, you get REM sleep behaviour disorder (Chapter 20.3).

REM is concentrated in the second half of the night, which is why cutting sleep short by two hours removes a disproportionate amount of it, and why alcohol — which suppresses REM early and causes rebound later — produces such broken second halves of the night.

What sleep does:

Memory consolidation. Slow-wave sleep consolidates facts and events; REM appears to handle procedural learning and emotional processing (Chapter 11.6). Learning something and then sleeping produces better retention than the same interval awake.

Emotional regulation. REM sleep appears to strip the emotional charge from memories while keeping the content — which is why a difficult event feels less raw after sleeping, and why sleep deprivation makes everything feel harder.

Immune function, metabolic regulation, and hormone release.

And the clearance described above.

How much you need

Seven to nine hours for most adults.

More for teenagers — 8 to 10 — and their body clock genuinely shifts later at puberty, which is a biological fact rather than laziness. Schools that start later show measurable improvements in attendance, grades and adolescent car accidents.

A very small number of people function on six hours because of specific rare gene variants. Almost everyone who believes they are one of them is not — the studies consistently find that people who sleep short show measurable impairment while reporting that they feel fine, which is exactly what you would expect when the faculty judging your performance is the one that is impaired.

Both too little and too much associate with worse outcomes, and long sleep is more likely to be a marker of other illness than a cause of harm.

Insomnia

Difficulty falling asleep, staying asleep, or waking too early, with daytime consequences, at least three nights a week for three months.

Around 10 percent of adults have chronic insomnia.

And the mechanism is worth understanding, because it explains why the standard advice makes things worse.

Something starts it — stress, illness, a new baby, shift work. That is acute insomnia and it usually resolves.

What makes it chronic is what the person does about it: going to bed earlier to catch up, staying in bed longer in the morning, napping, and lying awake trying to sleep.

Time in bed goes up, sleep does not, and so the proportion of time in bed spent asleep falls.

And then the bed itself becomes a cue for being awake and frustrated — the same conditioned learning as any other association. The person becomes sleepy on the sofa and wide awake the moment they get into bed, which is a symptom people describe constantly and rarely understand.

The treatment

Cognitive behavioural therapy for insomnia — CBT-I — is first line, and it outperforms sleeping tablets in the long term. Not "as good as"; better, because the effect persists after treatment ends.

Sleep restriction, and it is the most powerful component.

You limit time in bed to roughly the amount you are actually sleeping — if you are in bed nine hours and sleeping six, you spend six hours in bed. Sleep becomes more consolidated because sleep pressure builds. Then time in bed is extended gradually as efficiency improves.

It is uncomfortable for the first week and it works.

Stimulus control — rebuilding the association between bed and sleep. Bed for sleep and sex only. If you are awake for more than around 20 minutes, get up and go elsewhere until sleepy. Same wake time every day regardless of the night. No naps.

Cognitive work — addressing the catastrophic predictions about tomorrow, which are what convert tiredness into arousal. The worry about not sleeping is a bigger obstacle than the sleeplessness.

And digital CBT-I programmes have strong trial evidence, which matters because trained therapists are scarce.

Sleep hygiene alone — the familiar list about caffeine and screens — is not effective treatment for chronic insomnia. It is sensible background and it does not fix an established problem, and giving it as the whole answer is why so many people conclude that nothing works.

Sleeping tablets

Benzodiazepines and the "z-drugs" — zopiclone, zolpidem.

They work for a few weeks. Tolerance develops. Dependence follows. Rebound insomnia on stopping is worse than the original problem, which is what keeps people on them for years.

And in older people they increase falls, fractures and confusion measurably.

Short-term use in a crisis is reasonable. Long-term use for chronic insomnia is not, and it is extremely common.

Melatonin — most useful for circadian problems and jet lag rather than for ordinary insomnia, where the effect is small. Timing matters more than dose, and it is a body-clock signal rather than a sedative.

Sedating antihistamines — widely sold, tolerance within days, and a significant anticholinergic burden (Chapter 11.9).

Obstructive sleep apnoea

And this is the sleep disorder with the largest untreated burden, affecting perhaps a billion people worldwide with the majority undiagnosed.

During sleep the muscles of the throat relax. In some people the airway collapses, breathing stops, oxygen falls, and the brain briefly wakes to reopen it.

This can happen dozens of times an hour, every hour, all night — and the person has no memory of any of it.

Symptoms: loud snoring, witnessed pauses in breathing, gasping or choking, and unrefreshing sleep. Then daytime sleepiness, morning headache, poor concentration, irritability, and reduced libido.

And it is frequently the bed partner who reports it, which is worth knowing because the person themselves has no direct evidence of the problem.

Risk factors: obesity, a large neck, male sex, older age, alcohol, sedatives, nasal obstruction, and certain jaw and airway shapes — which is why it occurs in people who are not overweight.

The consequences are the reason it matters:

High blood pressure, and particularly blood pressure that resists treatment.

Increased risk of heart attack, stroke, atrial fibrillation and type 2 diabetes (Chapter 18.2).

And road traffic accidents — the risk in untreated sleep apnoea is several times higher, which is why it must be reported to licensing authorities in most countries and why treating it is a public safety matter, not only a personal one.

Diagnosis: a sleep study, at home or in a laboratory, counting the number of breathing interruptions per hour.

Treatment: CPAP — continuous positive airway pressure. A mask delivering pressurised air that holds the airway open, acting as a pneumatic splint.

When used properly it is close to curative — sleepiness resolves, blood pressure falls, and driving risk normalises. The whole problem is tolerating the mask, and getting the fit and the settings right is what determines success. People who abandon it usually do so in the first fortnight, which is exactly when support matters most.

Alternatives: mandibular advancement devices, which pull the lower jaw forward, effective for mild to moderate disease; weight loss, which genuinely improves it; positional therapy for people whose apnoea occurs only on their back; surgery in selected cases; and hypoglossal nerve stimulation, an implanted device for people who cannot tolerate CPAP.

And avoiding alcohol and sedatives in the evening, both of which relax the airway further.

Central sleep apnoea — different mechanism, where the brain fails to send the signal to breathe. Associated with heart failure and with opioids.

The other sleep disorders

Narcolepsy — loss of the hypothalamic neurons producing orexin, a signal that stabilises wakefulness, in most cases from an autoimmune process.

Excessive daytime sleepiness with irresistible sleep attacks.

Cataplexy — sudden loss of muscle tone triggered by emotion, usually laughter, which is essentially REM paralysis breaking into wakefulness and is close to diagnostic when present.

Sleep paralysis and vivid hallucinations at sleep onset, both of which are REM intruding on waking.

Diagnosed with sleep studies, and treated with stimulants or wake-promoting agents plus specific drugs for cataplexy. Average diagnostic delay is years, because the sleepiness is attributed to lifestyle.

Restless legs syndrome — an uncomfortable urge to move the legs, worse at rest, worse in the evening, relieved by movement.

Associated with iron deficiency, and checking ferritin and replacing iron is the first step and frequently effective. Also with kidney disease and pregnancy.

Treated with iron, then with alpha-2-delta ligands such as gabapentin. Dopamine agonists were standard and are now used more cautiously because of augmentation — the condition worsening and spreading over time on treatment.

REM sleep behaviour disorder — acting out dreams. The strongest known predictor of later Parkinson's disease and related conditions (Chapter 20.3). Managed with a safe bedroom environment, and with melatonin or clonazepam.

Circadian rhythm disorders — the internal clock out of step with the required schedule. Delayed sleep phase is common in adolescents and young adults, and it is a clock problem rather than a discipline problem. Treated with timed light exposure in the morning and timed melatonin in the evening.

Shift work disorder — and shift work is associated with increased cardiovascular, metabolic and cancer risk, which is worth knowing for anyone doing it long-term. Strategic light exposure, planned napping, and consistent scheduling reduce the harm.

Parasomnias — sleepwalking, night terrors, sleep talking. Arising from deep non-REM sleep, mainly in children, mostly outgrown. Managed by keeping the environment safe rather than by waking the person.

The practical version

Same wake time every day, including weekends. The wake time anchors the clock; the bedtime follows.

Morning daylight, which is the strongest signal the clock receives.

Caffeine has a half-life of around 5 to 6 hours, so an afternoon coffee still has a quarter of its dose in you at midnight.

Alcohol shortens sleep onset and wrecks the second half of the night, so it is a sedative rather than a sleep aid.

Cool, dark, quiet.

Do not lie awake in bed. Get up, do something dull in dim light, return when sleepy.

And exercise, which improves sleep quality reliably, at any time of day for most people.

One night of poor sleep is not a problem, and treating it as one is how chronic insomnia starts. The body catches up on the parts that matter most on its own.

What the next Part covers

Part 21 works through the remaining diseases worth knowing — the lungs, the gut and liver, the kidneys, the thyroid, the joints and the skin — each covered the same way: what starts it, what it feels like, and what is actually done about it.