What the Somnus Index measures
The 0 to 100 score Nightsong builds from your night: what goes into it, what it can honestly tell you, and how to read it against your own baseline.
Every morning, Nightsong greets you with 1 number: the Somnus Index, a score from 0 to 100 built from what your phone heard overnight. One number standing in for 8 hours is a serious compression, so it deserves a plain explanation. Here is what goes into the score, why a single number is worth having at all, and how to read it without giving it more authority than any number deserves.
What is a sleep and breathing score?
The idea is older than Nightsong. Most measurement tools that watch something long and messy, a night, a season, an economy, eventually publish a single index so people can follow it. The recipe is always the same: measure a few signals continuously, weigh them, and scale the result onto a fixed range.
A sleep and breathing score applies that recipe to the audible part of your night. Through the dark hours there is a stream of sound: breaths, stretches of silence, minutes of snoring, the occasional cough or passing car. The score condenses that stream into 1 value on a 0 to 100 scale, computed the same way every night, so that tonight can be compared honestly with the 30 nights before it.
The fixed recipe is the important part. A score computed one way on Tuesday and another way on Friday would be noise. Computed identically every night, it becomes a measuring stick, and a measuring stick is the thing a trend is made of.
What goes into the Somnus Index?
Two ingredients, both of them things a phone on your nightstand can genuinely hear.
The first is your minutes of snoring. What snoring is, physically, is soft tissue moving in moving air, and it arrives at the microphone as a distinctive, rhythmic sound that on-device analysis can count minute by minute across the night.
The second is the steadiness of your breathing. A settled night of sleep has a rhythm to it, breath after breath at an even pace, and that rhythm is audible. Nights differ in how even that rhythm stays, and steadier nights of breathing are one of the marks of restful sleep. Why steadiness is worth caring about is a subject of its own, and BetterSleepMetrics has a good plain-language piece on it.
Both measurements happen entirely on your phone, all night, with nothing sent anywhere. In the morning the 2 ingredients roll up into the single number, and the recording stays where it was made.
Why roll a night into 1 number?
Because trends are the whole point, and trends need something simple to be made of.
Thirty nights of raw audio is unreadable. Thirty nights of detailed charts is a hobby. Thirty points on a line is something you can take in with coffee in about 4 seconds, and taking it in regularly is what makes the data useful. The score is deliberately small so the pattern can be large.
There is a cost to the compression, and it is worth naming. Two different nights can land on the same number, and the number alone will not tell you which kind of night you had. That is fine, because the score was never meant to be read alone. It is meant to be read in a row, against your own baseline, where its movement means something.
What does a good score look like?
A good score looks like your usual score.
Bodies run on different settings, and ordinary nights contain different amounts of sound from person to person. That is why there is no universal target number, and why comparing your Somnus Index to a partner’s or a friend’s tells you almost nothing. The fair comparison, the only fair comparison, is you against your own recent past.
So the reading that matters is departure. Nights landing inside your usual band are your usual nights, whatever the absolute value. A slow drift away from your own normal, week after week, is the signal worth noticing, far more than any single low morning. Spikes have stories. There was a party, a cold, a brutal work week. Drift has causes.
Why did the number change overnight?
Because nights change, and they are supposed to. The conditions that shape the audible night move around: which way you were lying, how close the last drink was to bedtime, the state of your nose, how deeply a tired body let go. The mechanics are laid out in what snoring really is and the levers you can actually pull are in evenings that lead to quieter nights.
That variability is exactly why one night is never the story. A single surprising score is a question, not an answer, and usually the question is just: what was different about yesterday evening?
What can the score not tell you?
A fair amount, and honesty about this is part of reading it well.
The Somnus Index is an estimate built from sound. It cannot see you, weigh anything, or know why a night went the way it did. It describes your nights; it does not explain them, and it makes no judgment about your health. Nightsong is a general wellness tool, and the score is built for 1 job: showing you your own pattern clearly enough to learn from it.
The pattern can still be a good prompt. If every single night runs loud and mornings feel exhausted no matter how long you slept, that combination is worth a conversation with a clinician, and a clean record of your own trend is a useful thing to bring along.
How should you read it?
Lightly, on a schedule, with curiosity.
A morning glance to see whether the night landed in your usual band. A weekly look at the line, once, with coffee. Decisions on the month, never the night. And when you want to change something, change 1 evening habit at a time and let 2 weeks of scores grade it, which is the experiment mindset that turns a score from commentary into knowledge.
Held that way, the Somnus Index stays what it was built to be: a small honest number, computed privately on your own phone, that lets a month of nights speak plainly.
Further reading
Questions people ask
Is the Somnus Index a medical score?
No. It is a general wellness number that describes your nights against your own baseline. It does not measure or treat any condition, and it is not a substitute for a conversation with a clinician.
What is a good Somnus Index score?
There is no universal good number. Bodies differ, so the score is built to be read against your own baseline. A steady line at 62 and a steady line at 80 can both be perfectly ordinary nights for the 2 people producing them.
Does my sleep audio leave my phone?
No. The recording and every step of the analysis stay on your phone. The Somnus Index is computed there each night, and you own every night.
How many nights until the score means something?
Averages start to steady after roughly 3 to 7 nights, and about 2 weeks of ordinary nights gives your baseline time to settle. Before that, hold single scores lightly.
Why did my score drop after an ordinary day?
Nights vary on their own. Position, a late drink, a stuffy nose, or an unusually deep night after a short week can each move the number. One low score inside a steady month is a data point, and the month matters more.