24.7 — Sleep, Rest and Stress

You can survive weeks without food and days without water.

Rats deprived entirely of sleep die within two to three weeks — sooner than from starvation.

Sleep is not a gap between the parts of your life that matter. It is when a large amount of the maintenance happens (Chapter 20.11) — memory consolidation, waste clearance from the brain, hormone release, tissue repair, and immune consolidation.

And it is the pillar people trade away first, because unlike food and exercise, its absence produces no immediate visible cost.

This page is the practical side: what sleep loss actually does, how to sleep better, and how stress fits alongside it.

What sleep loss does

And these are measured effects, not warnings.

Cognition. After 17 to 19 hours awake, performance on attention and reaction tasks is comparable to a blood alcohol level of around 0.05 percent. After 24 hours, comparable to around 0.10 percent — over the drink-drive limit almost everywhere.

Which is why drowsy driving is a genuine road safety problem and why several countries treat it as seriously as drink driving.

Memory. Sleep before learning prepares the brain to encode; sleep after learning consolidates. A night of poor sleep reduces the ability to form new memories measurably.

Emotion. Sleep deprivation increases amygdala reactivity to negative stimuli by a large margin, and weakens the prefrontal control over it (Chapter 11.6). Which is the mechanism behind being irritable, reactive and less able to see things in proportion when tired.

Appetite. Short sleep raises ghrelin and lowers leptin, increasing hunger and specifically the preference for energy-dense food. Trials of sleep restriction show increased calorie intake.

Metabolism. Even a few nights of restricted sleep measurably reduces insulin sensitivity in healthy young people.

Immunity. People sleeping under 6 hours are several times more likely to develop a cold after controlled exposure to a virus than those sleeping more than 7. Vaccine responses are reduced by sleep restriction around the time of vaccination.

Cardiovascular. Short sleep is associated with hypertension, heart disease and stroke.

And a striking natural experiment: hospital admissions for heart attack rise measurably in the days after the spring clock change, when an hour of sleep is removed from a whole population at once.

Long-term: short sleep is associated with increased dementia risk, and the glymphatic clearance mechanism gives a plausible reason why (Chapter 20.11).

How much you need

7 to 9 hours for most adults.

Teenagers 8 to 10, with a body clock that genuinely shifts later at puberty.

Children more, and infants a great deal more.

And the point from Chapter 20.11 worth restating: almost everyone who believes they function fine on 5 hours is wrong. Studies of sleep-restricted people find performance declining steadily while self-rated alertness plateaus — the faculty judging your performance is the one impaired.

The genuine short sleepers with specific gene variants are rare — well under 1 percent.

How to tell if you are getting enough: you wake without an alarm most days; you do not feel sleepy in the afternoon; you do not need to catch up substantially at weekends; and you fall asleep in 10 to 20 minutes rather than instantly.

Falling asleep the moment your head hits the pillow is a sign of sleep deprivation, not of being a good sleeper.

The two systems that control sleep

And understanding these makes every piece of sleep advice make sense.

Sleep pressure — the homeostatic drive.

Adenosine accumulates in the brain throughout the day, and its build-up creates the feeling of sleepiness. Sleep clears it.

Caffeine works by blocking adenosine receptors — it does not remove the adenosine, it hides it. Which is why the tiredness arrives all at once when the caffeine wears off: the adenosine was accumulating the whole time.

The circadian rhythm — the body clock.

Governed by the suprachiasmatic nucleus in the hypothalamus, which runs on a cycle of slightly over 24 hours and is reset daily, primarily by light (Chapter 12.2).

Light striking specialised retinal cells signals the clock. Darkness allows melatonin release from the pineal gland, which signals biological night.

Good sleep happens when both systems align: high sleep pressure at the time your clock says night.

And every sleep problem is one of those two being out of place. Napping late reduces pressure. Late light delays the clock. Jet lag desynchronises them.

Sleeping better

The things that matter most

A consistent wake time, seven days a week.

This is the highest-value single change, and it is the one people resist. The wake time anchors the clock; the bedtime follows. Sleeping in at weekends produces social jet lag — effectively flying two time zones every Friday and back on Monday.

Morning light. 10 to 30 minutes of outdoor light soon after waking is the strongest signal the clock receives. Outdoor light on an overcast day is many times brighter than indoor lighting.

Evening darkness. Dim the lights in the last hour or two.

And on screens: the effect of blue light is real and smaller than the effect of what you are doing on the device. An engaging, stimulating or stressful activity delays sleep more than the light does. Night mode helps a little; putting the phone down helps more.

Caffeine timing. Half-life of around 5 to 6 hours, so a quarter of a 2pm coffee is still circulating at midnight. A cut-off 8 to 10 hours before bed is a reasonable rule, and sensitivity varies considerably with genetics.

Alcohol. It shortens the time to fall asleep, suppresses REM sleep in the first half of the night, causes rebound and fragmentation in the second half, and worsens sleep apnoea. It is a sedative, not a sleep aid.

Exercise. Improves sleep quality reliably. For most people, exercising in the evening is fine — the old advice against it does not hold up — though very intense exercise in the last hour before bed disturbs sleep for some.

A cool bedroom. Core temperature must fall by around 1 degree to initiate sleep. Around 16 to 19 degrees suits most people.

And a warm bath or shower 1 to 2 hours before bed helps for a slightly counter-intuitive reason: it brings blood to the skin, and the subsequent heat loss accelerates the core temperature drop.

Dark and quiet. Blackout curtains, an eye mask, earplugs, or steady background noise.

The bed rules

Bed is for sleep and sex only.

No working, no scrolling, no television. You are building an association, and the association is what makes you feel sleepy on getting into bed rather than suddenly alert (Chapter 20.11).

If you are awake for more than about 20 minutes, get up. Go to another room, do something dull in dim light, and return when sleepy.

Lying awake trying to sleep is the single most counterproductive thing you can do, both because effort prevents sleep and because it teaches the bed to mean frustration.

And do not clock-watch. Turn the clock away. Knowing it is 3:47 adds arithmetic about tomorrow to an already awake brain.

Naps

A 10 to 20 minute nap improves alertness without grogginess.

Longer naps risk waking from deep sleep, producing sleep inertia — the heavy, disoriented feeling that can last half an hour.

A 90 minute nap completes a full cycle and avoids that.

Nap before 3pm, because later naps reduce sleep pressure for the night.

And a nap is not a solution to chronic insufficient sleep, though it is genuinely useful for shift workers and for anyone facing a long drive.

The coffee nap — caffeine immediately before a 20 minute nap — works, because caffeine takes around 20 minutes to act and arrives just as you wake, on top of a partly cleared adenosine load.

Shift work and jet lag

Shift work disrupts the clock, and it is associated with increased cardiovascular, metabolic and cancer risk with long exposure.

What helps: bright light during the shift; dark glasses on the journey home; a completely dark, quiet, cool bedroom; consistent sleep timing even on days off where possible; forward-rotating shifts (morning to evening to night) rather than backward; and strategic naps before a night shift.

Jet lag — roughly a day per time zone to adjust naturally.

Eastward is harder, because advancing the clock is harder than delaying it — the natural rhythm runs slightly longer than 24 hours, so delaying comes easily.

What helps: shift your schedule by an hour or two in the days before travel; get light at the right times — morning light when travelling east, evening light when travelling west; take melatonin at the destination's bedtime, where it is genuinely effective for jet lag specifically; and adopt local meal times immediately.

Stress

And it belongs on this page, because sleep and stress are the two halves of the same system.

What stress actually is

A response, not an event.

Acute stress: the sympathetic nervous system fires, adrenaline is released, heart rate and blood pressure rise, blood is diverted to muscle, and glucose is mobilised (Chapter 12.4).

This is useful, and it is designed to be brief.

Chronic stress: cortisol stays elevated.

And sustained cortisol produces the harm — suppressed immune function, visceral fat accumulation, raised blood pressure and glucose, disturbed sleep, and effects on the hippocampus that impair memory.

The distinction that matters: acute stress is adaptive, and chronic unresolved stress is what damages.

What actually reduces it

Exercise. The best-evidenced stress intervention there is — it metabolises the stress response rather than leaving it circulating.

Sleep. Which is circular, and it is why breaking the loop at either end works.

Social connection. One of the strongest protective factors for both mental and physical health (Chapter 24.8).

Time outdoors. Time in green space is consistently associated with reduced stress markers.

Slow breathing. Extending the exhalation activates the parasympathetic system through the vagus nerve (Chapter 11.9), which physically slows the heart. Around 6 breaths a minute. It works within a minute and it is available anywhere.

Mindfulness and meditation — reasonable evidence for stress, anxiety and depression, with effect sizes that are modest and real.

Control and predictability. The single best predictor of whether a stressor causes harm is how much control the person has over it. This is why the same workload is damaging in one job and tolerable in another.

Purpose and meaning, which are associated with better outcomes across a wide range of measures.

And saying no, which is not a wellness slogan but the only intervention that reduces the input rather than managing the response.

What does not

Alcohol. Reduces anxiety for a few hours and raises it markedly the next day, while wrecking sleep.

Ignoring it.

And working harder to get out from under it, which is the most common response and rarely the effective one.

When to seek help

For sleep: insomnia lasting over 3 months; loud snoring with witnessed pauses in breathing (Chapter 20.11); falling asleep during the day despite adequate sleep; acting out dreams; and any sleep problem affecting daily function.

And CBT for insomnia is first-line treatment and outperforms sleeping tablets long-term, with digital programmes available where therapists are not.

For stress: when it is persistent, when it is affecting sleep, relationships or work, when it is producing physical symptoms, and when low mood or anxiety are present (Chapters 20.7, 20.8).

The short version

Same wake time every day. Morning daylight. Dim evenings. Cool dark bedroom. No caffeine after early afternoon. Alcohol is not a sleep aid. Do not lie awake in bed. And treat 7 to 9 hours as a requirement rather than an aspiration.

For stress: move, sleep, breathe out longer than you breathe in, stay connected to people, and get some control over the thing that is causing it.

None of that is new, and all of it works.

What the next page fixes

Chapter 24.8 covers ageing — what actually happens as we get older, what genuinely extends a life, and what the evidence says about the many things claimed to.