Why The Apple Watch Is So GOOD!

This is the first Apple Watch I ever

reviewed, the Apple Watch Series 6. But

is it still any good five generations

later? Well, we'll find out. I'll put it

through my current set of systematic and

scientific tests, and we'll see how it

stacks up against the competition. This

is actually the first Apple Watch with

the current generation heart rate

sensor, which is the same one that we

find in the Apple Watch Series 11. Now,

for several years, this Apple Watch

Series 6 had been gathering dust in my

drawers with all the other watches that

I've tested. But that actually made me

wonder, since relatively little has

actually changed in terms of sensors,

will this old hardware still perform as

good as the newer Apple Watches if I

upgraded to the latest firmware? So,

that's exactly what I did. Luckily, it

still charged up without any issues and

was able to use it. But of course, what

we actually want to know and test is if

the health and sports tracking

capabilities of a quite old Apple Watch

are still up to modern standards. So,

let's take a look. By the way, for those

of you that are new to the channel, my

name is Rob and I'm a post-doctoral

scientist specializing in biological

data analysis. Now, regular viewers will

know we'll start with indoor cycling

because it's one of the easiest

exercises for devices to track where we

have the reference device, which is the

Polar H10 EG chest strap along the

horizontal axis, which can generally

record my heart rate very accurately and

the Apple Watch Series 6 along the

vertical axis. And if both would agree

perfectly, all points should be on or

close to blue line. And indeed, they are

on or close to the line except for a few

points here below it. But it's really

just a few. You can also see that based

on this high correlation value which is

rounded to 1.00 that is this R value up

here. This cannot be higher than one. So

rounded value of 1.00 is just very good.

But let's make sure there are no

dropouts because I could imagine with an

older watch this is even a bigger issue

than with some of the modern ones. Now I

did a total of eight spinning sessions

and this is the first one where we have

the reference in blue green and the

Apple Watch Series 6 in red. And in this

first spinning session we have almost

perfect agreement with some minor

deviation potentially right here. But

this is as good as it gets. This second

session also looks really good. Some

minor deviation here and also right

here. This dip wasn't fully detected but

otherwise very good. This third spinning

session is also almost perfect. And we

see the same thing here for this final

example I want to share. So all of them

just look really good. So for indoor

cycling, the older Apple Watch Series 6

is still amazing. However, as you will

see later on, there are a few issues

with some of the other exercises. But

let's first compare this performance to

that of many other devices I've tested

in the past. And that's displayed in

this overview right here. So we have

that correlation value along the

horizontal axis. That basically means

that the further to the top right the

device is, the better is its agreement

with the reference device. And the Apple

Watch Series 6 is marked in red right

here. So, it's really among some of the

best performers, though maybe slightly

lower than some modern Apple Watches.

But let's zoom in a little bit. So, this

is that zoomed in view, which are just

the devices of a correlation of 0.9 or

higher. And amongst all of these, the

Apple Watch Series 6 is still among some

of the best performers. Though I do

wonder if the more modern ones like the

Apple Watch Ultra 3, the Apple Watch

Ultra 2, the Apple Watch Series 9, the

Apple Watch Series 10, the Apple Watch

Series 11 are doing a tiny bit better.

It's a small difference and probably

doesn't really matter. All of them are

good enough, but potentially there's

already a small difference between this

older watch that I retested now and some

of the newer devices. But we can

actually check that by looking at some

of the exercises that are typically

harder to track. And let's start with

running. And those running results are

right here, which honestly still look

really good. So the correlation is still

rounded to 1.00

and almost all points are on or again

close to the blue line. So unless there

are dropouts, there's nothing much to

complain about. But we can see that by

looking at the runs themselves. Now here

we have the first test run that I did,

which looks pretty good or really good

actually. There's some minor deviation

like right here at the peak of my heart

rate, but only by a few BPM. Otherwise,

all intervals were correctly detected.

So, nothing to complain about. And also,

this session right here looks almost

perfect. This doesn't do any worse, I

think, than even the Apple Watch Ultra 3

that I recently tested. But again, we

can compare to the competition, which is

displayed right here. So, again, we want

the devices to be as far to the top

right as possible. And the Apple Watch

Series 6 is doing just as well as some

of these other devices. Let's actually

zoom in a bit. So, these are just the

devices with a correlation of 0.9 or

higher. And the Apple Watch Series 6

does just as well basically as the Apple

Watch Series 11, Apple Watch SE 2025,

Apple Watch Series 9, and the Apple

Watch Ultra 2 and three. I think all of

these are doing really well and I can't

really distinguish the performance of

any of them based on the testing I've

done. So, even that older Apple Watch

Series 6 is doing really well, but as

you'll see in a second, for biking, I

had some more issues. Now, running this

channel next to my full-time job as a

scientist isn't easy or cheap for that

matter. For instance, this month I spent

more than €2,000 on just Apple watches

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be greatly appreciated. But let's get

back to the results. And here we have

that cycling overview, which does still

look very good because we have a

correlation of 0.98 and almost all

points are on or close to the blue line.

There is a bit more deviation. Now,

there's especially a few points below

the blue line, but as you'll see in the

individual bike rides, that's where the

main problem is. It's not huge, but it's

there. Now, I did a total of 17 bike

rides, I believe. And you can see that

the Apple Watch Series 6 here in red.

Sometimes it's actually too low heart

rate, but usually there are dropouts

around these times. So, it only measured

my heart rate a few times right here

during this peak, and it was too low. It

mostly realized it couldn't accurately

detect my heart rate. So, it stopped

detecting my heart rate. But there's

definitely more deviation than I'm used

to. During this ride right here, it

actually missed my heart rate for part

of the ride. Now, I'm not sure if this

was an auto detected one, which could

have worse detection. And as you can

see, there's quite some dropout moments

here. Now, as I said, this might be an

auto detected one, which would be

countered a bit differently, but this

doesn't look great. Then again, this

session is pretty good with some minor

deviations. Also, right here, looking

pretty good though. Sometimes there's

potentially some short dropouts, but

also for this session right here, for

instance, we see quite some dropouts in

the beginning. This again looks pretty

good, but again, a dropout. And also

right here in the beginning, some minor

dropouts. And also for this session

right here, there's quite some

deviation. Now, again, in these moments,

it only detects my heart rate a few

times, which means it somehow realizes

the signal quality isn't very good. And

this is why there were only a few points

below the blue line in the overview. But

it does mean that there are some issues

with my particular Apple Watch Series 6.

I don't know what's going on because

here, for instance, it isn't quite as

bad, but still not as good as we've seen

from my testing of some more recent

Apple Watches. And I don't know if this

is degradation over time or some other

issue. But I did use the most recent

watch OS 26 firmware. I did use the beta

firmware for a while, but this didn't

seem to impact some other Apple watches

I tested around the same time. Now, if

we just purely look at the correlation,

we can again compare this to the

competition. And purely

correlationbased, it's still doing

really well. So, it's still among some

of the top performers. We can actually

zoom in a bit again. So, here we have

the Apple Watch Series 6, which is

really close to, for instance, the Apple

Watch Series 8, the Apple Watch SE 2022.

So, still close to some Apple watches,

but it's potentially doing worse than

some more recent Apple watches like the

Series 11, Series 10, Ultra 2, Ultra 3.

Now, I do have to do more testing for

these newer devices, but somehow there

does appear to be a small difference

between my testing of the Apple Watch

Series 6 and the newer devices. But

again, I'm not sure if this is somehow

random chance, somehow degradation of

the device over time, or if it really

somehow performs a little bit worse and

the hardware was improved without us

actually observing this. But to close

things off, let's take a look at the

results for weightlifting, which is

typically the hardest exercise for a

device to track. And mostly as we sort

of expect for Apple watches, the

correlation is actually still quite good

at 0.99 and most points are on or close

to the blue line. Though you can see

that in the higher heart rate range,

there's more deviation than in the lower

heart rate range. What I do expect is a

lot of dropouts here, especially because

we already had them for biking. I expect

even more during weightlifting. So,

let's take a look. And for this first

weightlifting session, we don't see

terrible dropouts, but basically at the

peak of my heart rate, often there were

some minor dropouts. So I wouldn't

really trust my heart rate here during

weightlifting. And also here in the

beginning, there was a quite big

dropout. So really not great. And during

this second weightlifting session, we

again have clear dropouts, especially at

the peaks of my heart rate. And it also

struggled right here where was probably

moving around a little bit. And also

this final weightlifting session shows

signs of dropout. So a lot of the peaks

in my heart were not fully detected or

even wrongly detected. So for

weightlifting, I wouldn't use a watch in

general. Probably an earbased heart rate

tracking device or an ECG chest strap is

the best way to go. For instance, the

Apple AirPods Pro 3 did really well in

my initial testing for weightlifting. So

that's probably better than any watch.

Apple actually says that you can wear

both your AirPods Pro 3 in addition to

an Apple Watch and it will just select

the best heart rate signal. I still need

to test that, but that does seem like a

good way of going about things, but as I

said, I don't know how good it is at

prioritizing each signal. So, overall, I

would say the heart rate tracking of the

Apple Watch Series 6 is still among some

of the best devices out there, but it

does somehow seem like it showed more

issues than the testing that I did with

more modern Apple watches. the more

recent Apple watches, even the normal

Apple Watch line like the Apple Watch

Series 9, 10 or 11. So, I would be

really curious to know about your

experience as well. Have you noticed

degradation over time with your Apple

Watches? Or maybe you've switched from

an older Apple Watch to a newer one and

you've noticed improved or maybe even

decreased performance. If you have any

of those experiences, let us know in the

comments below. But let's next take a

look at another important thing,

especially given that Apple Watch

recently launched their sleep score,

which is still available on the Apple

Watch Series 6. Let's look at the sleep

stage tracking. Can the Apple Watch see

when you're in deep sleep, light sleep,

or REM sleep? Or specifically, can the

Apple Watch Series 6 see this? Let's

take a look. And here we have an

overview of the sleep stage tracking

results. We used the ZMAX EEG device as

a reference, which was specifically

designed for sleep tracking, is used in

several scientific studies, and actually

uses brain waves, eye movements, and

other things to track my sleep stages.

It's not perfect, but it's the best

reference I've got, and it's good enough

to give us a general impression of which

watches are good at sleep stage tracking

and which aren't as good. And the Apple

Watch is a good sleep stage tracker, and

the Apple Watch Series 6 is also still a

good sleep stage tracker. So we have the

reference device on top, the Apple Watch

Series 6 on the left, and if they would

perfectly agree, all values on this

diagonal right here in yellow should be

100%. So first of all, we see that about

89% of what was deep sleep according to

the reference was also deep sleep

according to the Apple Watch Series 6.

So that's really good. And any confusion

in this case was with light sleep. Light

sleep agreement is also very good at

about 83% and confusion being with any

of the other sleep stages. And finally,

RAM sleep stage tracking is also pretty

good at about 72% with most confusion

being with light sleep. Now, RAM sleep

is typically the hardest stage for many

devices to track. I don't know exactly

why, but they really struggle and it's

one of the easier stages to track with

an EEG headband because with eye

movements, you can quite clearly see

much of the RAM sleep. This is looking

pretty good. I'm quite happy with these

results. It's in line with what we've

seen for some other Apple devices, but

we can actually put these results into

the context of those other devices, and

that will really help us in

understanding how good or bad this

actually is. And here's an overview of

many of the devices I've tested in the

past with the average agreement over

those three sleep stages we just looked

at along the horizontal axis and the

agreement of the worst out of those

three sleep stages along the vertical

axis where all devices tested against

the ZMAX EG headband. So the one we used

in this video marked in green and we

have the Apple Watch Series 6 right

here. Now, the better the agreements at

the reference, the more to the top right

the device is. And the Apple Watch

Series 6 is among some of the best

performers. Now, there are currently

five brands that have the best sleep

stage tracking out of any device. Those

are the Apple Watch and no matter which

Apple Watch you get, you basically get

the same sleep stage tracking, which is

quite good, at least based on my

testing. There's the Pixel Watch. Since

2 months or so, that has improved sleep

stage tracking. That's really good. and

it likely means all Fitbit devices also

have improved sleep stage tracking.

Third, we have the Aura Ring, which is

also a really good sleep stage tracker.

There's the NUA or Sleep 2 app. And

finally, there's the Eight Sleep Pod.

So, all five of these devices have

roughly equal sleep stage tracking

performance, at least in all my testing.

Now, my favorite device out of these

five for my sleep personally is the

Hleep Pod, and that's because it both

has good tracking, but it also improves

my sleep. Now, the HLED Pot isn't a

wearable like the Apple Watch, but it's

something that goes around your

mattress, and it can actively heat and

cool each side of the bed independently,

so you and your potential partner can

sleep at different temperatures. And for

me, this has been a godsend. In summer,

I really need a lot of cooling down of

the bed. And the H pot has been the one

reason I was able to sleep on those hot

summer nights. And when it gets colder,

I like to have my bed heated a little

bit before getting into bed. and

especially partners I've had in the past

really like to sleep a lot warmer than I

do so they can set their temperature

independently and I can still sleep

comfortably at the temperature that I

want. Now, it's quite expensive, but if

you want the best discount possible, use

my affiliate link up here or down here

and you'll be supporting the channel at

the same time while helping yourself

potentially get the best sleep possible.

But back to the Apple Watch. The Apple

Watch is also really great if you're

just interested in sleep stage tracking.

At least that's what all my testing so

far has shown. And that's true for any

Apple Watch you get because during the

night measuring raw data like movement,

heart rate, breathing rate are

relatively easy no matter which Apple

Watch you have and is the algorithm that

is most important. So it translates

these different things mostly movement

in the case of Apple to the sleep

stages. So that's why usually all brands

are in roughly the same position in this

graph because it's the algorithm that is

more important than the hardware. But

these are the results. Again, these are

the results on me. So there might be

some variation from person to person.

But as I've shown in scientific

literature in another video which I'll

link below, generally the results are

very similar to my testing. So the sleep

stage tracking is still one of the best

out there. And if you're still rocking

an Apple Watch Series 6, 7, or even an

older Apple Watch, as long as the new

sleepstaging is supported, I suspect

you'll be getting some of the best sleep

stage tracking out there. But what about

the GPS tracking performance? I expect

this might have improved a lot over the

last few years. So, let's see how the

Apple Watch Series 6 did for cycling and

running when it comes to GPS tracking.

Now, the way I test the GPS tracking

performance is by cycling the exact same

route several times. And if the signals

overlap very well, that likely means

that the GPS tracking performance is

pretty good. However, if they deviate a

lot from each other, we can say with

some certainty that the GPS tracking

performance likely isn't that great. And

here we have the results for four times

a cycle to work using the exact same or

more or less the same route. And I

always start my route on the corner

right here. And at least the GPS signals

are quite quickly, though not very

accurately in the beginning. But let's

see if it gets better. So, we have quite

some deviation here, even going through

some buildings. Then here, it's looking

okay. Still quite some deviation here.

We also have a lot of deviation. Still

not looking good. The signals just

aren't very consistent. They're all over

the place. So, it might be I use

different routes here, though. I don't

suspect that's actually the case. This

doesn't look very good compared to the

competition. Usually here, the results

are much more consistent. Also right

here, a lot of deviation. Still a lot of

deviation. So this first test doesn't

look very good compared to more modern

smart watches. But let's now take a look

at four times I cycled back from work

and then some running results. So now

we're looking at the results for four

times I cycled back from work. One time

actually starting from a different

location. So the signals were again

acquired quickly. So that's good. But we

want to know about the consistency

between those signals. This actually

looks quite good. Here we have more

deviation than I would like to see. This

also doesn't look very good. This one

signal is just all over the place. Also

a lot of deviation here. I cannot really

go through the middle of this

roundabout. So that doesn't look very

good. Also here, this signal is quite

bad. Here we also have a lot of

deviation. And basically up until the

end, the signals deviate a lot from each

other. Much more than modern Apple

watches. At least that's the feeling I'm

getting so far. I'll be doing a full

test of the Apple Watch Series 11 soon.

So then we'll see if that's at least

better. We know already that the Apple

Watch Ultra 3 that I recently did a full

test for is definitely better than this.

And that actually has dualband GPS now.

So that makes some sense. But I even

think that the Apple Watch Series 10,

for instance, did do better than this.

But let's now take a look at the results

for running. And those results for

running are right here. So I run the

same circle many times right here in

part Belvadier. And again, we want good

consistency between the signals, but

they're really not that consistent. It's

really all over the place. There's a lot

of deviation between the signals. Also,

right here, one signal just goes out

away from the others quite a bit. And on

this side where there's a big wall right

here, it struggles even more. The

signals are really all over the place.

So, somehow this older Apple Watch

really is struggling. I can actually

show you the comparison to the Ultra 3.

So here we have a similar run, not the

exact same one, but a similar run I did

with the Ultra 3. And the signals are

super close together. You can almost not

see the individual lines until I zoom in

more. And zooming out, it actually looks

quite good. And if you try to take the

same perspective with the Apple Watch

Series 6, this looks a lot worse. If I

go back and forth, this is the Ultra 3.

This is the Series 6. You judge which

you think is better, but I think it's

pretty obvious. So, what it seems like

is that the Apple Watch Series 6, at

least on me, didn't have good GPS

tracking anymore. I don't know for sure

if the hardware somehow degraded over

time, but the results speak for

themselves. If you have an older Apple

Watch, potentially you're getting less

good GPS tracking. So, taking all that

together, given that this is quite an

old Apple Watch, it hasn't been used in

years, it did really well compared to

the competition. It outperformed most

watches out there. Weirdly though, there

were some minor quirks with the heart

rate tracking having more dropouts than

I expected and also the GPS tracking

wasn't amazing. I do get the impression

that newer Apple watches that I have did

perform better in both those regards,

especially the GPS tracking wasn't

amazing on my Apple Watch Series 6. If I

compare it especially to the Apple Watch

Ultra 3, it's really a night and day

difference. However, if we just care

about sleep stage tracking and heart

rate tracking, this Apple Watch Series 6

still really outperforms most of the

competition, which is saying something

that after more than 4 years, companies

still need to catch up with this older

hardware. Of course, the firmware has

improved over time because back when

this watch was released, you couldn't

even get sleep stages with the Apple

Watch Series 6. So, a lot of

improvements can also be made with just

firmware updates. Now, some of these

differences between more modern Apple

watches and this older Apple Watch

Series 6, I cannot really explain. I

don't know if there's actually any

degradation in performance just because

it was lying around for so long, not

being used. I'm not an expert in these

kinds of things. I can just show you how

the performance is right now. And I hope

it made some sense to you and I would

also be really curious in your

experience. Do you have an older Apple

Watch and does it still work well for

you? We've also seen that for instance

older Apple Watch SE which even have an

older generation heart rate sensor and

then the Apple Watch Series 6 still

perform really well. So I think Apple

just has a good combination of

integrating their hardware with their

firmware and giving you reliable

results. Now, if you do end up getting

an Apple Watch, like the new Apple Watch

Series 11 or the Apple Watch Ultra 3, a

Pixel Watch, maybe an Aura ring, a Whoop

strap, an eight sleep pot, or anything

at all on Amazon for that matter, even

something as small as toilet paper, you

want to get the best discount possible,

and at the same time support the

channel, there are different affiliate

links in description below. Now, given

that you watched this whole video on the

Apple Watch, I think you will like this

video on Apple's new sleep score or this

video on the HLE pod.