How WHOOP 4.0 measures health metrics and expands wearability

Originally published on September 8, 2021

WHOOP 4.0 changed continuous health monitoring by adding more places to wear the sensor, more physiological signals, and more on-device hardware in a smaller form factor. In Episode 139 of the WHOOP Podcast, WHOOP Founder and CEO Will Ahmed and WHOOP Co-Founder and Chief Technology Officer John Capodilupo explain how WHOOP 4.0 was built, what new measurements it introduced, and why its design decisions mattered for day to day performance and health insights.

This article breaks down six of the most important changes from that launch: WHOOP Body, SpO2, skin temperature, Health Monitor, Sleep Coach with haptic alarms, and the sensor upgrades behind better signal quality. It also explains the engineering tradeoffs Capodilupo describes, from motion artifact and sensor placement to battery chemistry and wireless charging.

Note: This article covers WHOOP 4.0. For the latest hardware, see the current WHOOP membership page.

For the full launch discussion with Ahmed and Capodilupo, listen to Episode 139 of the WHOOP Podcast on Spotify.

What does WHOOP Body change about where you can wear WHOOP?

WHOOP Body changed the system from a wrist-first wearable into one that could work in several locations around the body. The point was not style alone. It was signal quality, comfort, and the ability to keep collecting data during activities where the wrist is not always the best place for a sensor.

Capodilupo explains that the core challenge was motion artifact, which is the movement of the sensor relative to your skin. WHOOP measures heart rate with light based sensing called photoplethysmography, or PPG. When the sensor shifts, the optical signal can get noisy. That noise can come from blood movement and tendon movement, but Capodilupo says the largest source is often the sensor itself moving against the skin. Earlier WHOOP research had already shown that the bicep could outperform the wrist for some activities, so the team expanded the question from one alternate placement to many possible ones.

That pushed the product far beyond a new strap. WHOOP engineers tested sensor performance in garments, including bras, boxers, shorts, and leggings, while the apparel team worked on how fabric, compression, placement, and pod design would hold the sensor close enough to the body for consistent readings. Capodilupo describes this as a joint problem between hardware, algorithms, and soft goods, not a single design choice. The result was WHOOP Body and the broader idea of Any-Wear placement, which you can also see summarized in the WHOOP Unlocked 2021 recap.

Capodilupo said the team tested much more than a wrist and a bicep:

"We've mapped out 10 to 20 different locations and done similar data collections."

That statement matters because it shows how the feature was validated. The team was not guessing. Capodilupo says test subjects were wearing multiple WHOOP prototype sensors and multiple prototype garments at the same time so engineers could compare raw signals from different body sites. That gave WHOOP a path to narrow down the locations that could support dependable data quality.

The larger takeaway is that placement became part of the product platform. A smaller sensor made WHOOP Body possible, but the real unlock was building algorithms and garments that could preserve performance outside the wrist.

What you should take away

• WHOOP Body was built to improve wearability and support strong optical signals in more than one body location.
• Motion artifact, or sensor movement relative to the skin, is a major reason wrist based heart rate sensing can get noisy.
• WHOOP tested 10 to 20 body locations before narrowing down the placements used in WHOOP Body products.
• Sensor placement, garment design, and algorithms all had to work together for off wrist wear to perform well.

Once WHOOP had more viable places to wear the sensor, the next question was what additional signals the smaller hardware could measure.

How does WHOOP 4.0 measure SpO2 and skin temperature?

WHOOP 4.0 added SpO2 and skin temperature to broaden what the hardware could observe during day to day wear and overnight monitoring. SpO2 gives insight into blood oxygen saturation, while skin temperature adds another signal that can help show when the body is deviating from its typical pattern.

Capodilupo describes SpO2 as an extension of the same optical sensing approach WHOOP already used for heart rate. Instead of looking at beat to beat frequency, the system uses red and infrared light and measures how much light is absorbed and reflected back through the skin. Oxygenated and deoxygenated hemoglobin absorb those wavelengths differently, which is why pulse oximetry works.

On the podcast, Capodilupo framed the baseline most people are familiar with: healthy blood oxygen saturation is generally in the 95 to 100 percent range. He also pointed to two use cases that made the metric especially relevant in 2021. One was altitude adaptation for athletes. The other was daily health context during the COVID-19 period, when many people became familiar with finger pulse oximeters.

Capodilupo explained the mechanism plainly:

"Oxygenated hemoglobin and deoxygenated hemoglobin absorb red and infrared light differently."

Skin temperature required a different clarification. WHOOP 4.0 measures the temperature at the surface of your skin, not core body temperature. Capodilupo was explicit about that distinction. Core temperature is what a thermometer measures in the mouth or a clinician measures during an office visit. A wrist based sensor measures a peripheral signal, but its nightly changes can still be useful. In the episode, Capodilupo says those fluctuations can help monitor wellness or illness status and also support more research into female physiology.

That tie to female physiology is important. Skin temperature changes across the menstrual cycle, so adding this signal created a path for better cycle aware insights over time. For members already familiar with behavior and physiology logging in the WHOOP Journal, this new signal made that picture more detailed.

If you want to hear Capodilupo explain the optical mechanism behind SpO2 in his own words, listen to Episode 139 of the WHOOP Podcast on Spotify.

What you should take away

• WHOOP 4.0 measures SpO2 with red and infrared light, not with the same green LEDs used for heart rate alone.
• SpO2 helps show how much oxygen your blood is carrying and can add context for altitude adaptation and health monitoring.
• WHOOP 4.0 measures skin temperature at the skin surface, not core body temperature.
• Nightly changes in skin temperature can add context for wellness status and future female physiology insights.

With more signals available, WHOOP needed a way to show members when those numbers moved away from their usual range.

What does WHOOP Health Monitor actually do?

Health Monitor gives WHOOP members a single view of key physiological signals and alerts when those signals move outside their typical range. Instead of showing isolated readings, it uses each person’s baseline as the frame of reference.

Ahmed describes Health Monitor as a new dashboard inside the WHOOP app that watches for deviations in resting heart rate, heart rate variability, respiratory rate, SpO2, and skin temperature. On the episode, he also gives one clear threshold: SpO2 should stay above 94 percent, and Health Monitor can alert you when it drops below that level. The larger point is not a single universal cutoff. It is that WHOOP can compare your current numbers with your normal pattern.

Capodilupo says this was a direct response to a simple problem: most people only see these metrics once a year at a routine physical. With continuous collection, WHOOP can show the trend every day, which makes it easier to spot when something is off for you. Ahmed connected that idea to the PGA Tour case of Nick Watney, whose elevated respiratory rate preceded a positive COVID-19 test. WHOOP later featured that story in the WHOOP Unlocked 2021 recap.

Capodilupo summarized the logic of Health Monitor this way:

"Instead of just seeing if you're above or below an absolute threshold based on a wide demographic survey, you can really understand what's normal for your body and get alerted when you're not in that normal range anymore."

That is the section of the launch that most clearly connects hardware to daily decision making. Heart rate variability, resting heart rate, respiratory rate, SpO2, and skin temperature all become more useful when viewed as an integrated baseline rather than a stack of disconnected numbers. WHOOP was already built around personalized metrics such as Recovery and Sleep need. Health Monitor extended that same baseline driven logic to a wider set of vitals.

There is also a practical behavior angle. If you are at altitude, for example, an SpO2 alert may reflect the environment rather than illness. Health Monitor does not replace clinical care, but it can tell you your body is outside its normal state and that the day may call for different choices.

What you should take away

• Health Monitor brings resting heart rate, heart rate variability, respiratory rate, SpO2, and skin temperature into one view.
• WHOOP uses your baseline range to decide when a metric is out of the ordinary for your body.
• A daily baseline is more informative than a once a year office reading when you are trying to spot meaningful deviations.
• Health Monitor can add context to training, recovery, travel, altitude exposure, and illness related changes.

From there, the next hardware question was how WHOOP could act on sleep need, not just report it.

How do Sleep Coach and haptic alarms work?

WHOOP 4.0 added a haptic motor so the device could wake you with vibration instead of sound. That motor powered a Sleep Coach experience built around three wake options: an exact time, a sleep goal, or a wake when your Recovery reaches the green range.

Ahmed calls the third option the one many members would find most interesting. If you choose the in the green mode, WHOOP can wake you once your body has recovered into the green, while still respecting the latest time you are willing to wake up. That turns the alarm from a fixed clock into a recovery aware decision rule.

Capodilupo says the hardware sounds simple but was difficult to execute because the wrist is not very sensitive. The motor had to be strong enough to wake people up, but stronger motors also draw more energy. That meant the haptic system had to fit within the same battery budget as the added sensors and smaller form factor.

Capodilupo explained the design constraint this way:

"The wrist is actually one of the least sensitive parts of your skin."

That single point changes how you think about haptic design. A small buzz that feels obvious on another part of the body may be too weak on the wrist. WHOOP tested multiple vibration motors and balanced three competing needs: wake strength, size, and battery consumption. Ahmed says the team ultimately chose a larger motor because it had to function as a real alarm, not just a notification.

This section of the launch also shows how WHOOP thinks about sleep as planning rather than passive tracking. Sleep Coach was already designed to estimate how much sleep you need based on recent Strain, sleep debt, and goals. Haptic alarms gave that recommendation an action layer. The system could now help you wake at the point that best matched the rule you selected.

For Ahmed and Capodilupo’s full explanation of Sleep Coach modes and the haptic motor tradeoff, listen to Episode 139 of the WHOOP Podcast on Spotify.

What you should take away

• WHOOP 4.0 added a haptic motor so the device could wake you with vibration instead of a sound based alarm.
• Sleep Coach introduced three wake modes: exact time, sleep goal, and in the green.
• Wrist sensitivity is low, so the motor had to be strong enough to wake people without draining too much battery.
• Haptic alarms turned Sleep Coach from a planning tool into a feature that could act on the sleep rule you chose.

Adding more hardware usually increases size or reduces battery life, so the next part of the launch was about how WHOOP avoided both outcomes at the same time.

How did WHOOP 4.0 get smaller and keep multi-day battery life?

WHOOP 4.0 became 33 percent smaller than WHOOP 3.0 while keeping a multi day battery life by combining lower power electronics, more efficient firmware behavior, tighter hardware packaging, and a new battery chemistry. It also redesigned the battery pack around wireless charging so the charger could become waterproof.

Ahmed says size was a years long argument inside the product process because every added feature pulled against battery life and internal space. Capodilupo describes the inside of the device as a Tetris problem. The team had to fit more sensing, a haptic motor, and a redesigned optical stack into a smaller package. Ahmed adds that WHOOP 4.0 contains about 200 components, and even a change of a tenth of a millimeter mattered during industrial design reviews.

Capodilupo points to two main battery strategies. The first was aggressive power management. Components were selected for low power draw, and the processor spent much of its time asleep between bursts of work. The second was a partnership with Sila Nanotechnologies, which developed a commercially deployed silicon anode battery material that increased energy density for the same physical battery volume.

Capodilupo gave one of the clearest engineering specifics in the episode:

"The algorithms and everything has to run so efficiently because we only want it to supply power maybe only like 20 milliseconds out of every second."

That detail shows how battery life is preserved. Continuous monitoring does not mean every component runs at full speed every moment. It means the system wakes, samples, processes, and sleeps in a tightly controlled cycle.

The battery pack redesign matters just as much for daily use. Previous battery packs were a common pain point because water exposure could ruin them. WHOOP 4.0 solved that by charging the sensor wirelessly from the pack instead of using exposed electrical contacts. Capodilupo says that change made it possible to give the battery pack the same waterproof rating as the strap. Ahmed also notes a small but useful update: a double tap on the battery pack shows the remaining charge level. The WHOOP 4.0 and WHOOP Body announcement includes additional launch details on the hardware package.

What you should take away

• WHOOP 4.0 became 33 percent smaller than WHOOP 3.0 despite adding new sensors and a haptic motor.
• Battery life depended on low power components, efficient firmware behavior, and a processor that spent much of the time asleep.
• Sila Nanotechnologies battery materials increased energy density, which helped WHOOP fit enough power into a smaller battery.
• Wireless charging let WHOOP make the battery pack waterproof and remove exposed charging contacts from the sensor design.

Even with better wearability and battery life, the launch would have meant less without a stronger optical signal, which is where the sensor redesign became central.

How did WHOOP improve signal quality and sensor accuracy?

WHOOP improved signal quality in 4.0 by redesigning the optical sensor from the ground up, increasing the number of LEDs and photodiodes, and validating the new setup across large human data collections. The goal was cleaner signals in real conditions, not just better readings in a lab mockup.

Capodilupo says WHOOP had largely kept the same sensor configuration from Generation 2 to Generation 3, but 4.0 started fresh. The team built about 90 different sensor configurations to test how LED count, LED brightness, photodiode count, geometric layout, and the mechanical structure on the bottom of the device influenced signal to noise ratio. That process involved bench work, but it depended mostly on empirical studies with real participants.

The final hardware changes were concrete. WHOOP 4.0 used three green LEDs instead of two, added red and infrared LEDs for SpO2, and increased the number of photodiodes from one to four. More detectors and more flexible wavelengths gave the algorithms more information to work with. Capodilupo says WHOOP also completed more than 20,000 data collections in development and planned to keep improving algorithms after launch.

Capodilupo put the scope of the redesign in one quote:

"We actually built around 90 different sensor configurations in different prototype WHOOP straps, looking at how different LEDs and photodiodes, the light detectors, could affect the signal to noise ratio."

The testing process mattered almost as much as the hardware itself. Capodilupo says each prototype round involved recruiting 20 to 40 people, then comparing sensor output against reference devices such as chest straps and electrocardiograms during controlled protocols and sports activity. Ahmed also highlights a factor many people miss when they think about optical sensing: participant diversity. WHOOP wanted the sensor to work across a wide range of skin tones and wrist anatomies, so those variables were part of recruiting and evaluation from the start.

This section also explains why WHOOP treats a hardware release as a platform rather than an endpoint. Capodilupo says the hardware team and WHOOP Labs planned new algorithm work after launch so the same sensor system could improve over time. If you want background on the longer WHOOP approach to strain, recovery, and sensing, the earlier article What is WHOOP? gives useful context.

What you should take away

• WHOOP 4.0 redesigned the optical sensor to maximize signal to noise ratio rather than making a small cosmetic update.
• The team built about 90 sensor configurations before choosing the final LED and photodiode layout.
• WHOOP 4.0 increased the optical stack to three green LEDs, one red LED, one infrared LED, and four photodiodes.
• More than 20,000 data collections and diverse participant testing helped validate the sensor in real use conditions.

The Bottom Line

• WHOOP 4.0 expanded wearability by supporting validated body placements beyond the wrist, including placements enabled by WHOOP Body apparel.
• SpO2 in WHOOP 4.0 uses red and infrared light to estimate blood oxygen saturation based on how oxygenated and deoxygenated hemoglobin absorb light differently.
• Skin temperature in WHOOP 4.0 measures the temperature at the skin surface, which can add useful nightly context without representing core body temperature.
• Health Monitor became the place where resting heart rate, heart rate variability, respiratory rate, SpO2, and skin temperature could be viewed against a personal baseline.
• Sleep Coach with haptic alarms added three wake modes: exact time, sleep goal, and wake when Recovery enters the green range.
• WHOOP 4.0 became 33 percent smaller than WHOOP 3.0 while preserving multi day battery life through lower power design and a denser battery material.
• Wireless charging made the WHOOP 4.0 battery pack waterproof and removed exposed charging contacts from the sensor design.
• WHOOP improved optical signal quality in 4.0 by testing about 90 sensor configurations and increasing the sensor stack to five LEDs and four photodiodes.

Frequently asked questions about things discussed in this episode

How does WHOOP 4.0 measure SpO2?

WHOOP 4.0 measures SpO2 with red and infrared light that detect differences in how oxygenated and deoxygenated hemoglobin absorb light. WHOOP then uses that optical information to estimate blood oxygen saturation during regular wear.

What does WHOOP do with skin temperature data?

WHOOP uses skin temperature trends as an added context signal for changes in your normal physiological state. WHOOP measures temperature at the skin surface, especially during sleep, rather than claiming to measure core body temperature from the wrist.

How does WHOOP Health Monitor know when something is off?

WHOOP Health Monitor compares current readings with your typical baseline range rather than relying only on a broad population threshold. WHOOP can then flag when resting heart rate, heart rate variability, respiratory rate, SpO2, or skin temperature moves outside your normal pattern.

What does WHOOP Sleep Coach do with haptic alarms?

WHOOP Sleep Coach uses the haptic motor in WHOOP 4.0 to wake you by vibration based on the rule you choose. WHOOP can wake you at an exact time, when you hit your sleep goal, or when your Recovery reaches the green range.

How does WHOOP support wearing the sensor outside the wrist?

WHOOP supports more body placements by pairing a smaller sensor with garments and algorithms designed for those locations. WHOOP tested multiple placements and narrowed them to positions that could preserve signal quality in real movement.

What changed in the WHOOP 4.0 battery pack?

WHOOP 4.0 changed the battery pack to wireless charging, which helped make the charger waterproof. WHOOP also added a double tap battery level check so you can see whether the pack has enough charge before sliding it onto the sensor.

How did WHOOP improve sensor quality in 4.0?

WHOOP improved sensor quality in 4.0 by redesigning the optical stack with more LEDs, more photodiodes, and extensive prototype testing. WHOOP built about 90 sensor configurations and validated the final design across large human data collections.

WHOOP 4.0 showed how better sensor placement, better optical design, and better baseline context can turn a smaller device into a more informative health and performance tool.