A field guide to your body

Your body, decoded.

You don't need more data. You need to understand what you already have.

Twelve numbers your wearable captures every day. What they actually mean, in plain English — and the small habits that move them.

The most valuable thing in healthcare isn't a prescription. It is a pattern.

Every person is different. The same number is high for one body and low for another. So we wrote this as a guide, not a verdict — a plain reading of what each metric is, what healthy looks like, and what you can do about it.

01

Heart Rate Variability.

The pulse between your pulses.

What it is

HRV is the tiny variation in time between one heartbeat and the next, measured in milliseconds. A metronome heart sounds steady — but it is actually a stressed one. A flexible heart is a resilient one.

Why it matters

It is the clearest real-time window into your nervous system. Rising HRV means you are recovered and ready. Falling HRV means your body is still under load, whether you feel it or not.

A healthy signal

Rises or holds steady week over week.

A warning signal

A sudden drop that persists for several days — travel, illness, overtraining, poor sleep, grief.

Move the needle

  • Sleep by the same time each night
  • Train easy on easy days, hard on hard
  • Breathe out longer than you breathe in
  • Reduce alcohol in the evening

02

Resting Heart Rate.

Your body at rest. And what it says about everything else.

What it is

The number of times your heart beats per minute while your body is completely still — usually first thing in the morning.

Why it matters

A lower resting heart rate means a stronger heart doing less work. It is one of the oldest and most honest markers of cardiovascular fitness.

A healthy signal

Between fifty and seventy for most adults. Athletes often sit between forty and fifty five.

A warning signal

A slow upward drift over weeks — often a sign of poor sleep, illness on the way, or chronic stress.

Move the needle

  • Thirty minutes of easy cardio, most days
  • Quality sleep before anything else
  • Less caffeine after noon
  • Breathwork, even five minutes a day

03

VO2 Max.

The single best number for how long you'll live.

What it is

The maximum volume of oxygen your body can pull in and use during hard exercise, per kilogram of body weight.

Why it matters

Study after study has found VO2 Max to be the strongest predictor of how long you live — stronger than blood pressure, stronger than smoking, stronger than diabetes.

A healthy signal

Moving in the right direction year over year. Above average for your age.

A warning signal

A steady decline that isn't explained by aging or injury.

Move the needle

  • One or two hard interval sessions per week
  • Keep most of your training easy
  • Stay consistent for months, not weeks
  • Strength train twice a week

04

Sleep Stages.

You aren't asleep. You are working.

What it is

Your night breaks into four stages — awake, core, deep, and REM. Each one does a different job for a different part of you.

Why it matters

Deep sleep rebuilds your body. REM consolidates memory, emotion, and learning. Without enough of each, everything else you do in the day works a little less well.

A healthy signal

Around fifteen to twenty percent deep. Twenty to twenty five percent REM. Minimal time awake.

A warning signal

Fragmented sleep, waking often, sleep that leaves you groggy no matter how long you slept.

Move the needle

  • A cool, dark bedroom
  • No screens the last hour
  • Same wake time, even on weekends
  • No alcohol within three hours of bed

05

Sleep Debt.

Your body keeps a tab. It always collects.

What it is

The running gap between the sleep you needed and the sleep you got. It is measured in hours and it accumulates across days.

Why it matters

Short term debt shows up as slower reactions, worse mood, and higher injury risk. Long term debt raises the risk of almost every chronic disease we know of.

A healthy signal

Under two hours of rolling debt.

A warning signal

Seven nights of shortage — even when you tell yourself you feel fine.

Move the needle

  • Go to bed an hour earlier, not an hour later
  • Two consecutive longer nights beat one weekend binge
  • Short afternoon naps, never close to bedtime
  • Treat sleep like training — plan for it

06

Respiratory Rate.

Breath is a vital sign before anything goes wrong.

What it is

The number of breaths you take per minute while you sleep.

Why it matters

Your breathing rate at night is remarkably stable. When it shifts, something in your body is shifting too — often days before you feel the first symptom.

A healthy signal

Twelve to twenty breaths per minute. Steady from night to night.

A warning signal

A sudden rise of two or more breaths per minute, sustained over several nights.

Move the needle

  • Treat a rise as a soft warning
  • Eat well, sleep more, train less
  • Hydrate
  • Nasal breathing during the day

07

Blood Oxygen.

How well your lungs hand oxygen to your blood.

What it is

The percentage of your blood that is carrying oxygen, measured through your skin at the wrist.

Why it matters

When it drops, your body has to work harder for the same output. Consistent overnight dips can point to sleep apnea or other issues that are worth asking a doctor about.

A healthy signal

Ninety five to one hundred percent at rest.

A warning signal

Regular nightly readings below ninety percent.

Move the needle

  • If it is low overnight, talk to a clinician
  • Quit smoking
  • Stay physically active
  • Sleep on your side, not your back

08

Wrist Temperature.

Your body's thermostat runs on a cycle. It tells a story.

What it is

Your skin temperature at the wrist while you sleep, tracked relative to your own personal baseline rather than a universal number.

Why it matters

Small rises often appear before illness symptoms, during the second half of a menstrual cycle, or after a hard block of training.

A healthy signal

Stable within your personal band.

A warning signal

A half degree or more above baseline for two or more nights in a row.

Move the needle

  • When it rises, your body is doing work. Rest.
  • Don't chase a training session through it
  • Hydrate more
  • Pay attention to what else is happening

09

Recovery.

You don't get stronger when you train. You get stronger when you recover.

What it is

A composite reading — HRV, resting heart rate, sleep, respiratory rate — that estimates how ready your body is to take on load today.

Why it matters

Training on top of poor recovery makes you slower, not faster. Knowing where you stand lets you choose: push or protect.

A healthy signal

Green on most days, yellow on harder days, red only occasionally.

A warning signal

Three or more low days in a row — your body is quietly accumulating debt.

Move the needle

  • Sleep first
  • Easy cardio on low days
  • A deload week every four to six weeks
  • Learn to call a rest day a win

10

Strain.

How much your body actually worked today.

What it is

The total cardiovascular load from everything you did — training, walking, heat, stress, a long flight.

Why it matters

Not every hard day is a training day. Stress counts. Heat counts. Travel counts. Strain shows you the full bucket, not just the workout.

A healthy signal

Aligned with your recovery — higher strain on higher recovery days, easy days when you need them.

A warning signal

Seven days of rising strain with no rest.

Move the needle

  • Use strain to plan, not to judge
  • Match hard days to green recovery
  • Count the stressful days as training too
  • A rest day is an earned result

11

Heart Rate Zones.

Five gears. Five different bodies.

What it is

Five training zones, each targeting a different system — endurance, fat burning, threshold, VO2 Max, neuromuscular power.

Why it matters

Most people train too hard on easy days and too easy on hard days. Mixing the zones correctly is the quiet secret behind every elite training program.

A healthy signal

About eighty percent of your training is easy. Ten to twenty percent is genuinely hard. Very little in the grey zone in between.

A warning signal

Everything feels medium. Nothing feels truly easy or truly hard.

Move the needle

  • Walk, jog or bike slow enough to hold a conversation. That is Zone Two.
  • One short hard session per week
  • Keep the easy easy. That is the whole game.
  • Use a heart rate monitor, not feel

12

Vitality Age.

How old your body actually is.

What it is

An estimate of your physiological age, built from HRV, VO2 Max, resting heart rate, sleep quality and daily activity.

Why it matters

Chronological age is the calendar. Vitality age is the body. One of them you cannot change. The other one you can.

A healthy signal

Trending down, or flat, against your actual age.

A warning signal

Trending up by more than a year per year.

Move the needle

  • Sleep
  • Zone Two cardio
  • Lift twice a week
  • Whole food. Breath. Repeat.

You already have the data.

Now you know what to do with it.

Your Apple Watch, Oura, Whoop or Garmin captures all of this already. The numbers are only as useful as the habits you build around them.

Want to go deeper?

Long reads on the science behind each of the metrics above — written in plain English, with references to the research.

Heart Health

HRV, explained properly

→

Fitness

VO2 Max and longevity

→

Sleep

How much sleep do you actually need?

→

Recovery

The science of recovery

→
Browse every article →

Understand your body. Act on it.

Laso turns the numbers above into a simple daily read — what to do today, what to rest from, and why.

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