Where to start
Most people arrive with one of three questions. The first is about a number: is my resting heart rate normal, what does my HRV mean, how much deep sleep is enough. The second is about a habit: how do I sleep better, how do I recover after hard training. The third is about a purchase: which tracker is worth it, and what does it cost over time.
The guides below follow those three questions. If you have a number in front of you, start with normal resting heart rate or heart rate variability. If your question is about nights, start with sleep stages and deep sleep. If you are comparing devices, go to the comparison pages, which show dated prices read from each maker.
What a wellness band can and cannot tell you
A wrist or finger device senses motion, pulse, skin temperature and sometimes blood oxygen. From those signals it estimates when you slept, how your heart rate changed and how recovered you may be. Those are estimates for trends at home. A sleep laboratory records brain activity and eye movements, and an electrocardiogram in a clinic records the heart's electrical signal. A consumer device does neither in the same way.
That is why every page here separates a measurement from an estimate, and why none of them says that a device diagnoses, treats or prevents a disease. The FDA's general wellness policy separates products that support a healthy lifestyle from products whose claims relate to a disease.
A good habit with any tracker is to look at weekly averages instead of single nights, to compare yourself with your own history instead of with another person, and to match the numbers with how you feel.
How to read a number from a tracker
Start with the question behind the number. A resting heart rate that is higher than usual for a few days can follow a poor night, a hard week, illness, alcohol or stress, and it can also be an ordinary fluctuation. The guides explain these links where an institution describes them, and they never turn one reading into a verdict.
Check how the number was taken. A pulse counted by hand at rest, a chest strap during a run and an optical sensor on a finger overnight are different measurements. The guide on wrist and chest strap heart rate compares them, and the guide on measuring your resting heart rate shows how to take a clean reading.
Look at the direction over weeks. Brands compute scores such as sleep or readiness with formulas they choose, so a score of 80 on one app is not an 80 on another. What carries over is your own baseline and how it moves. If a reading worries you or comes with symptoms, talk to a health professional instead of waiting for the next night's data.
All guides in this topic
Sleep stages explained: light, deep and REM sleep through the night
The stages of a night, in order, and what each one does.
Sleep score: what it measures, what makers publish and what a good score means
What goes into a sleep score, and what no maker tells you.
How much sleep do you need? Hours by age, from newborns to adults over 65
Recommended hours for every age group, and how to check your own.
How to improve sleep quality: habits from the CDC and NIH that you can start tonight
The habits public health agencies recommend, and a plan to try them.
Sleep debt: what it is, how to calculate it in hours and how recovery works
How lost sleep adds up, and what research says about paying it back.
Heart rate zones by age: how to find your target range
A target heart rate chart by age, the math behind it, and where it falls short.
Heart rate recovery: what it is, how to measure it and what the research says
How fast your pulse falls after exercise, and what that number can and cannot tell you.
How to measure your resting heart rate: take your pulse at the wrist or neck
A step-by-step pulse method, the conditions that matter and what a sensor adds.
Normal blood oxygen level: what SpO2 numbers mean and when to get care
The SpO2 ranges NIH and FDA pages give, how far a pulse oximeter can be off, and when to call.
Smart ring vs fitness band: a dated comparison of price, battery and subscriptions
Finger or wrist? A table of what makers list, with dates.
Best sleep trackers: a dated table of rings, bands and a bed sensor
Ten sleep trackers, one dated table, no invented ranking.
Best fitness trackers without a subscription: what each maker lists
Which trackers bill once, which add optional plans, which need a membership.
Oura vs WHOOP: membership cost, battery and features side by side
Two sleep and recovery wearables, one table, every cell dated.
How to improve HRV: what the research links to heart rate variability
Sleep, exercise, alcohol and stress against the studies, with the limits of each.
How to improve VO2 max: what the training studies and guidelines say
Endurance, intervals and weekly amounts, with the studies behind them.
Readiness score explained: what Oura, WHOOP, Garmin and Fitbit say goes into it
The inputs each brand lists, dated, and why a score from one brand is not a score from another.
Chest strap vs wrist heart rate: how accurate is each, and when does it matter?
What validation studies found, their limits, and when a chest strap is the safer choice.
Best smart rings: what each maker lists for price, plan, battery and fit
A dated table of smart rings, with no ranking, and where Elio fits even though it is not a ring.
Best fitness trackers: what each maker lists, and how to choose by use
A dated table of bands and watches, and a way to choose by what you will do with it.
Words you will meet on these pages
- Resting heart rate
- Your heart rate when you are calm and still, counted in beats per minute.
- Heart rate variability (HRV)
- The variation in time between heartbeats.
- Non-REM and REM sleep
- The two phases of sleep. Non-REM has three stages, from light to deep sleep.
- Readiness or recovery score
- A number a brand computes from sleep, heart rate and other signals. Each brand uses its own formula.
- Blood oxygen (SpO2)
- The share of oxygen carried by your blood, often estimated by a light sensor.
- VO2 max
- The highest rate at which your body can use oxygen during hard exercise.
How these guides are made
Each guide is written for readers in the United States. It uses American units and sources such as the National Institutes of Health, the Centers for Disease Control and Prevention and other named institutions. Every figure carries a numbered source with the date it was read. Prices of devices are read from the maker's own page on the day written and dated in the table.
Health topics carry the line “Medical review in progress.” until a qualified reviewer has checked the page. The pages do not replace advice from a health professional. In a medical emergency in the United States, call 911.
Frequently asked questions
Are these guides medical advice?
No. They explain public information from institutions and describe what consumer devices estimate. They do not diagnose or treat anything. Talk to a health professional about symptoms or concerns.
Which guide should I read first?
Start with the guide that matches your question: a number you saw, a habit you want to change, or a device you are comparing. The “Where to start” section links each of them.
Can a tracker replace a sleep study or an ECG?
No. A sleep study records brain activity and eye movements, and an ECG records the heart's electrical signal. A wellness device estimates trends from other signals and is not a diagnostic device.
Where do the prices come from?
Comparison pages read prices from each maker's own page on the day written and print the date in the table caption. A price that could not be read is not printed.