How does Fitbit assign to someone a "Sleep Score" for a given span of 'day'?

[Victamon]

I'm guessing that in broad-strokes more total time and more deep sleep are each favorable to lesser, and also higher percentage of deep sleep is good---whereas increased proportion of light sleep and to lesser extent REM is negative. Is this accurate? And what about the finer brushtrokes (the specifics)?

What has me somewhat puzzled is the looseness of correlation between my total sleep time and the score. Although when I get 8+ hours of sleep recorded my score is usually at least decent (unsurprisingly), some days it records especially small amount yet with a score better than poor: just yesterday it recorded my total duration as measly 2h41m yet with a respectable score of 65(within 'Fair'). Why is this? I do feel like for thr most part the scores are accurate-ish, but as for the sleep duration.. meh. dunno, but I think I got more slumber than 3h!

Is it weighted by previous days' scores? Relatedly: how does it partition days (for sleep recordings/allocations)? especially considering polyphases.

[RiekoC]

Hi @Victamon 

This excerpt from What should I know about Fitbit Sleep Stages explains how your Fitbit device determines if you're asleep: 

Fitbit estimates your sleep stages using a combination of your movement and heart-rate patterns. When you haven’t moved for about an hour, your tracker or watch assumes that you’re asleep. Additional data—such as the length of time your movements are indicative of sleep behavior (such as rolling over, etc.)—help confirm that you’re asleep.

I believe you get a sleep score when your Fitbit records about 3 hours of sleep. If it recorded you being in REM/light sleep and decent Restoration during that time,  you could get a score around 65 since it calculates those as percentages of your sleep. I haven't read anything so far that suggests your sleep score is weighted by the previous days' scores. 

Like you, I have had nights where I know I was asleep but it wasn't recorded by my Fitbit. I've also had times when my tracker recorded me as asleep when I've been reading a book or playing games on my tablet. Although I know that I can edit the sleep log, I don't bother except to remove the erroneous extended sleep in the morning. 

On most days, I wake up before I'm ready to get up, then lay in bed not moving much, which gets recorded as REM or light sleep. Knowing that I can "fool" my Fitbit so it records me as sleeping, I consider the sleep data as interesting but don't rely on it to be extremely accurate. 

The most important thing is that you feel refreshed and rested after your sleep.