Topic: Sleep and light
Sleep, light and evolution: what the evidence says
Everything this site carries on sleep and light, gathered in one place: the book's chapters and where they say to start, every popular claim they grade, and the reference topics and terms behind them.
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Where does the book say to start on sleep and light?
Chapter 7: Light and Dark
Get outdoor daylight early in your day, roughly 10 to 30 minutes
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.Dim the last two to three hours before bed
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.Get children outdoors about two hours a day; less still helps
StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.
Chapter 8: Sleep
Get up at the same time every day, weekends included
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.Make the bedroom dark, cool and quiet, with the phone charging somewhere else
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.for dark, for sleep timingEmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.for cool, for the phone in adults, and for heart and metabolic effectsIf insomnia has lasted three months or more, ask for CBT-I before sleeping pills
StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.
Which popular claims about sleep and light hold up?
Is mouth taping safe, and does it work?
Unsupported, with a safety catch.
EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.Is sunscreen bad for you?
Against: the one trial runs the other way.
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.a single trial with few melanomasDo blue light glasses work for sleep?
Mostly unsupported; skip the glasses unless a clinician suggests them for a specific sleep problem.
StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.that the evidence for the glasses is weakDoes ten minutes of morning sunlight set your body clock?
Right direction, invented number. Get outside early, and stay longer when it is gray.
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.Should you wait 90 minutes after waking for coffee?
Untested, and the reasoning looks backward. Drink your coffee when you like it in the morning; the timing that matters is the hours before bed.
EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.for any benefit of waitingDo tart cherry juice and magnesium help you sleep?
Unproven. The juice adds sugar, and magnesium is a long shot, not a treatment.
EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.Can NSDR or yoga nidra replace lost sleep?
A pleasant way to rest, not a substitute for sleep.
EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.for small benefits
What does the body expect, and what does it get now?
The mismatch cards of the reference topics behind these chapters, as the library states them.
Topic 16: The Shrinking Face
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.The face, jaws, teeth and upper airway.
- What it expects
- Hard chewing from childhood, teeth that wear flat, a clear nose, little free sugar.
- What it gets
- Soft processed food, fork-cut mouthfuls, industrial flour and sugar, and more body weight pressing on the airway.
- What happens in the gap
- Jaws that fall short of their teeth (crowding, impacted wisdom teeth), retained overbites, widespread decay, and possibly, alongside weight, some sleep apnea.
Topic 19: The Largest Organ
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.Skin: pigment shield, vitamin D factory, barrier and habitat.
- What it expects
- Sun matched to its pigment, changing gradually with the seasons, mostly outdoors and often in shade.
- What it gets
- Pale skin under subtropical sun, dark skin at high latitudes, indoor lives and sudden, burning doses.
- What happens in the gap
- Skin cancer where sun outruns pigment; low vitamin D, and at the extreme rickets, where pigment outruns sun.
Topic 23: Sleep and the Dark You Have Lost
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.The circadian clock, set through melanopsin-carrying retinal cells, and the sleep it schedules.
- What it expects
- A bright day, a dark night, a falling evening temperature, and the same schedule tomorrow.
- What it gets
- Days under 100 lux indoors, evenings at 30 to 300 lux, a lit bedroom, a schedule that moves at weekends.
- What happens in the gap
- Sleep moves later and grows irregular, the melatonin night shortens, and peripheral clocks fall out of step.
Topic 24: The Indoor Eye
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.Emmetropization, the visual feedback loop that grows the eyeball to match its optics.
- What it expects
- A childhood of bright outdoor light, thousands of lux for hours a day, with things to look at near and far.
- What it gets
- Years of classrooms at a few hundred lux, and 87% of time spent inside a building.
- What happens in the gap
- The eye overgrows, distance blurs for good, and a stretched retina carries lifelong risk.
Topic 1: The 200-Million-Year Warm-Up
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.The mammalian operating system: warm blood, night-built senses, milk, late-added color vision.
- What it expects
- High energy costs paid for by effortful foraging; real darkness at night; changing temperatures; food judged by color and smell.
- What it gets
- Abundant calories, climate-controlled rooms, bright evenings, engineered food colors.
- What happens in the gap
- Sleep and light (Topic 23), temperature (Topic 25), food reward (Topic 11); even the body's thermostat is drifting, for unknown reasons.
Topic 9: The Invisible River
EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.The lymphatic system: self-pumping vessels, 500 to 600 nodes, drains around the brain.
- What it expects
- Kilometers of walking a day, shifting postures, regular nights of sleep.
- What it gets
- Unbroken sitting, long-haul seats, short nights and, for some, severe obesity.
- What happens in the gap
- Swollen feet, more clots on long journeys, lymphatic failure at very high BMI; after sleep loss, amyloid rises of uncertain meaning.
Topic 41: The Handrail II: Light, Sleep, Temperature, Nature
ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.The circadian clock, thermoregulation, and the stress-recovery loop, all driven by environmental signals rather than conscious effort.
- What it expects
- Thousands of lux by day, near-total darkness at night, a temperature that falls after sunset, occasional cold and heat, and daily hours outdoors.
- What it gets
- Under 1,000 lux of waking light in the week the campers spent at home, evenings in the 30 to 300 lux band the clock reads most steeply, a thermostat that never moves, and most of a life indoors.
- What happens in the gap
- The clock drifts later and flattens, sleep timing scatters, and systems tuned by temperature and daylight have nothing to tune against.
Terms
The chapters and reference topics on this page cite 280 distinct works, each listed on its own page and in the source register. Educational content, not medical advice; read the disclaimer.