So, Iíve been tracking my sleep with Fitbit Charge 4 for a little bit over a month.
I want to share what Iíve learnt.

I've been thinking about writing this post for some time. I am sure I forgot something, so maybe I'll add more later or I'll share some graphs if anyone is interested. Discussion/questions/disagreement is welcome.

Why Fitbit

For a long time, Iíd been considering somehow tracking my sleep but always got dissuaded by bad results of various sleep tracker. But the curiosity in me won eventually and I did a thorough comparison of my options.

Here are some links for anyone interested:
Oura ring - some nice hypnograms and easy to understand confusion matrix:
https://d1a0efioav7lro.cloudfront.ne...ality-2016.pdf
A comparison of various brands:
https://academic.oup.com/sleep/artic...saa291/6055610
Various Fitbit models:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6908975/
Fitbit Charge 2 and Alta HR:
https://jcsm.aasm.org/doi/full/10.5664/jcsm.8032

Conclusion: Fitbit is a little bit better than Oura ring and significantly better than other competition. It also cost three times less and can do more (exercise tracking).

Strengths

The ability to recognize sleep vs. awake. Not perfect but not biased either way. It can be fooled but it is isnít bad.
According to the research, it has a super high sensitivity (the ability to detect sleep, approx. 95%) but lower specificity (the ability to detect wake, 60-70%) and it is also good for telling how long it took to fall asleep (sleep onset latency).

Weaknesses

The biggest weakness is in detecting deep sleep. The sensitivity is somewhere around 50%.
It shows me very little deep sleep and it shows me deep sleep in times where either there shouldnít be any or when I know with certainty that I wasnít in deep sleep.
So I completely ignore this and look at the sleep staging as awake - non REM - REM.

REM tracking

It is surprisingly good for REM tracking. On my active nights, I wake up after most of my dreams and take notes (time, dream recall). The ends of my REM periods as shown by Fitbit are almost always corresponding to my notes.
There are some mistakes but rarely major (like completely missing REM period). The mistakes can be the beginning of the REM period (hard to confirm) or gaps in the REM periods where I remember a continuous dream.
On some days, it looks like everything makes sense and is correct. Sometimes, when my sleep is weird, it gets confused and I donít really trust it. I usually can tell, though, or at least I think I can.

Usefulness for lucid dreaming

Recall

It really helps with recall. It makes it easier to tell which dreams/memories belong to which REM period. Together with my notes, it allows me to reconstruct most of my night. On a good night, I remember something vague from the 1st period, nothing from the 2nd and a lot of details from 3rd and 4th (and possibly 5th, if it happens).
It also helps with understanding when I recall my dreams and when I donít. Even though the deep sleep stage is unreliable, itís evident that deep sleep just after a REM period is a recall killer. Awakening directly from REM helps a lot with recall. If I let it go because I am too sleepy and take the notes a little bit later (10-30 minutes later), I remember progressively less.

Learning more about my sleep

This is very helpful for battling insomnia, timing WBTB and LD techniques and more. Surprisingly, I realized that my sleep isnít exactly typical. My early REM periods arenít significantly shorter than the later periods. I get less REM than the benchmark for my age. And the last two hours of my sleep are often messed up. Also, my cycles arenít regular (no neat 90 minutes cycle) at all and they are different every day.
I tried to do my WBTBs earlier (after 4 hours) and it helped me a lot. Sometimes, I also set an alarm for one of the early periods, hoping for a quick DEILD, but I havenít been successful yet.
As every tracker user find out, we sleep less than we think. During 9 hours in bed, I typically spend 8 hours sleeping and 1 hour awake even if my sleep is decent and I don't try to lucid dream. If I add reading for a while, talking with my husband for a short time and falling asleep, I need to be in bed for 10 hours to get 8 hours of sleep. This isn't new to me, more like a confirmation.
The sleep score I get from Fitbit usually corresponds well with how I feel. Which mean I can use it to get enough sleep without oversleeping.

WILD timing

I realized that waking up naturally just after REM is bad timing for me. Even if it is around the 6th hour of my sleep, the gaps between my REM periods are just too long (60-75 minutes).
But Iíve had luck with spontaneous WILDs in the last hour of my sleep. I often drift in and out of sleep at that time (some conscious NREM probably too) and I basically ďwake upĒ just before the vibrations. I can also tell when the transition from light sleep to REM ďwantsĒ to happen but it usually wakes me up a bit too much.


Resetting the cycle?

It looks like a short awakening doesnít reset the cycle. Longer awakening usually also doesnít but if I canít relax enough/fall asleep deeply enough afterwards, the next REM period gets pushed back. My body just has this need for REM sleep and tries to get it when needed. I canít tell when it gives up. When I have insomnia, it feels like itís trying to get at least a couple of REM minutes periodically, and the moment my mind relaxes enough, I start dreaming.

NREM REM-like dreams?

NREM REM-like dreams should be relatively common. But so far, I havenít been able to confirm one. By confirm, I mean remembering a dream clearly at the time when Fitbit shows NREM. It happened maybe once or twice but on both of these occasions, the data looked wrong.
There could be some dreams like that just before the REM periods, these would get recalled together with my REM dreams, but I have no way how to know about those.
In any case, I would expect to see some signs of these more often but I just havenít.
My usual NREM dreams, if I notice them, are just weird thoughts. Sometimes, there are simple 3rd person view visuals.

Supplements

The only supplements I tried during this period were B6 and 5-HTP and I couldn't see any significant difference in my sleep staging graph.