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How sleep, the heart, muscle and fitness work, and how Aera works out each number it shows. Every article names the studies behind it and links to each one.

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The basics

How your sleep, heart and muscles work, from the ground up · 20 articles

Your watch

  1. How a watch measures youWhat can a light on your wrist really see?

Sleep

  1. How sleep worksWhat happens in each stage of the night?
  2. How much sleep you needIs eight hours right for you?
  3. Your body clockWhy do you feel awake at the same hours?

Heart

  1. Resting heart rateWhat does a low resting heart rate say?
  2. Heart rate variabilityWhy can an uneven heartbeat be a good sign?

Recovery

  1. RecoveryWhat does it mean to be recovered?
  2. StressWhen is stress good for you?

Muscle

  1. How muscle growsWhat does lifting do to muscle, day by day?
  2. StrengthWhy does strength matter more with age?

Fitness

  1. VO2 maxWhy do doctors care how fit you are?
  2. IntensityHow hard is hard?
  3. Training loadHow much training is too much?

Everyday

  1. StepsHow many steps are enough?
  2. CaloriesWhere do the calories you burn go?

Vitals

  1. Breathing rateWhat does your breathing do while you sleep?
  2. Blood oxygenWhat do the numbers on your wrist mean?
  3. Wrist temperatureWhy does your skin warm up at night?
  4. Blood pressureWhat do the two numbers mean?

Age

  1. Biological ageCan your body be younger than you are?

How Aera works

What every number in the app means, and how it is made · 18 articles

Start here

  1. Your usualWhat counts as normal for you?

Sleep

  1. Sleep scoreHow does a night become one number?
  2. Sleep need and debtHow does Aera know what you owe?
  3. Bedtime, regularity and daylightWhat do bedtime, regularity and daylight show?

Recovery

  1. Recovery scoreHow does Aera turn your night into Recovery?
  2. Stress scoreWhy is your Stress score high today?
  3. EnergyHow full is your battery, really?
  4. Strain scoreHow hard was today on your heart?

Training

  1. Room to trainHow much can you train today?
  2. Workout effortHow hard was that session?
  3. Heart rate zonesWhere do your zones come from?
  4. Cardio loadHow hard was your week, really?
  5. Heart driftDoes your heart work harder late in runs?
  6. Fitness trendsHow does Aera call a fitness change?

Health

  1. Blood pressure averageHow is your blood pressure average made?
  2. Body ageHow do watch signals become one age?

Your data

  1. Where your numbers come fromWhere does your data come from?
  2. What stays on your phoneDoes your health data leave your phone?

Go deeper

Questions with answers you might not expect · 15 articles

Sleep

  1. Six hours of sleepDo six-hour nights cost you?
  2. Regular sleepDoes regular sleep beat long sleep?
  3. Weekend lie-insDoes a lie-in make up for lost sleep?
  4. Afternoon coffeeDoes your afternoon coffee cost you sleep?
  5. LamplightDoes evening light delay your body clock?
  6. Bright daysDo bright days and dark nights matter?
  7. A week of campingDoes a week outdoors reset your body clock?
  8. Sleep in the wildDid people without electricity sleep longer?
  9. Alcohol and sleepDoes a drink cost you sleep?

Heart

  1. The first minute after exerciseHow fast should your heart settle?
  2. An irregular pulseShould an irregular pulse alert worry you?

Fitness

  1. 220 minus your ageIs 220 minus your age a good guess?
  2. Any running at allDoes any amount of running help?
  3. The taperHow long, and how much, to taper?

Strength

  1. Light weightsDo light weights build as much muscle?