Sleep types

My VA3 does a reasonable job of telling me how much I sleep, compared to what I observe on an ordinary clock. I have no idea what it categorizes as "deep sleep" and "light sleep," and what significance these measures actually have. And what would I do with the data, even it were reliable, and open to reasonable interpretation?

There are situations where sleep depth needs to be measured in a medical laboratory setting. I'm not really sure why a healthy person would need to know this, or what such a person would do with the information.

I have no idea what it categorizes as "deep sleep" and "light sleep,"

Less wrist movement is interpreted as deep sleep, more wrist movement is interpreted as light sleep.

and what significance these measures actually have.

None, because wrist-based sleep measurement is more like guessing than real measurement.

And what would I do with the data, even it were reliable, and open to reasonable interpretation?

There are situations where sleep depth needs to be measured in a medical laboratory setting. I'm not really sure why a healthy person would need to know this, or what such a person would do with the information.

Sleep - being a very individual matter - is a tricky beast for any practical analysis. You could resort to some literature or a sleep specialist. Recently I saw a book about the impact of sleep on athletic performance, written by a medical expert of one of the pro cycling teams. Proper sleep analysis could help to make some amendments to achieve more efficient sleep, but I believe that data provided by consumer-level fitness devices are in most cases not enough to be of any use.

I'm not really sure why a healthy person would need to know this, or what such a person would do with the information.

Purely out of interest!

Less wrist movement is interpreted as deep sleep, more wrist movement is interpreted as light sleep.

If that's what it's based on then that's probably why I get the figures I do then. I'm quite a heavy sleeper and (my wife tells me) quite immovable when I'm asleep, even if I'm right over on her side of the bed!

Thanks for the answer

Less wrist movement is interpreted as deep sleep, more wrist movement is interpreted as light sleep.

Sure, and this is how Garmin explains it. But "less" and "more" are not, by themselves, useful measures. Less compared to what? To what extent (if any) does wrist movement correlate with sleep depth information you might get from, say, EEG traces?

But, broadly, I agree with you. I suspect that sleep depth information from a wrist-worn device will be for entertainment, like a newspaper horoscope.

Less compared to what?

I don't know. My observation is that garmin uses some form of relative comparison technique, meaning that for one user a certain level of movement might fall into the light sleep range, while for others it might fall into deep sleep range. It could even happen, that when you re-edit your sleeping time manually, Garmin Connect will rearrange sleep phases to different effect than before editing.

To what extent (if any) does wrist movement correlate with sleep depth information you might get from, say, EEG traces?

The theory is following:
https://en.wikipedia.org/wiki/Actigraphy
However there are studies that show that consumer-level wrist-worn fitness devices are closer to random results than real sleep phases.

But "less" and "more" are not, by themselves, useful measures.

That's because they are – logically, or by definition – not measures, not metrics, not thresholds and not scalar values.

Less compared to what?

Compared to more! If you must get all precise about it, less has a shorter distance from the mathematical mean than more, in whatever scalar variable (i.e. metric) that is being expressed as a one-dimensional number line. Whatever the particular metric is, or how it is derived from physical measurements, does not need to be known or specified, in order for less and more to have semantic meaning that can be understood by the average person. For all intents and purposes, that metric is regarded as having logical equivalence to the user's wrist movement, in Garmin's design framework.

To what extent (if any) does wrist movement correlate with sleep depth information you might get from, say, EEG traces?

x% correlation with y% confidence, where (x+y)<100.

Whatever the particular metric is, or how it is derived from physical measurements, does not need to be known or specified, in order for less and more to have semantic meaning that can be understood by the average person.

Being able to assign semantic values to "more" and "less" is not at issue here; the problem is establishing any meaningful distinction between the categories "deep" and "light" sleep as presented by Garmin.

What conceivable use is it to me, to know that at 4am my wrist was moving "more" than it was at 2am, when "more" is not quantifiable? There's absolutely nothing practical I can do with that information.

If I cared about my sleep -- and, frankly, I don't really -- I would want to know that the "deep" and "light" sleep categories presented by Garmin had some relationship -- even an approximate one -- to my neurological level of consciousness. I don't care about how much my wrist moves in my sleep, and I suspect that nobody else does either, except to the extent that it is an indicator of some more useful metric. If it is, then Garmin should state what that metric is, and how closely its categorization captures it. If it doesn't, then Garmin should desist from presenting meaningless data as if it were significant.

@LarsTheBear: Nobody said the sleep ‘information’ presented is useful, or somehow ‘actionable’ for the purposes of better understanding or management of the device user's behaviour or condition. Those who are so inclined can nevertheless use the information in the Check step of some sort of Plan-Do-Check-Act cycle, even if there is no scientific rigour (or even logical validity) in employing the information as some equivalent measure of the desired outcomes from the Plan step.

If someone actually cared about their neurological level of consciousness, then they should be wearing some sort of brain wave activity monitor on (or inside) their head, not a wrist-worn device that is (probably) cheaper, more convenient and less uncomfortable; even middle-schoolers would know the brain is nowhere near the wrist, and there is no circuit for brain waves to travel down to the wrist to be measured there. Nevertheless, clever and effective marketing is often based on allowing people to draw (or accept) ill-founded logical equivalence between what is on offer and actual benefits, without making false claims about a product or service directly.

Garmin should desist from presenting meaningless data as if it were significant.

I suspect Garmin will persist in presenting this data as long as people demand it and are ready to pay for it.