How the Somnus Index works, night by night
What happens between Start and your morning number: on-device listening, honest checks, Partial nights, and why some mornings show no number at all.
Published
Every morning, Nightsong™ puts one number at the top of Home: your Somnus Index™. This page walks through what happens between tapping Start and reading that number, and, just as important, what happens when the night does not give the phone enough to work with. If you want the bigger picture of what the score means, start with what the Somnus Index measures. This page is about the machinery.
Step one: the phone listens
You plug the phone in, set it on the nightstand or the mattress edge within a few feet of your head, and tap Start. After a night or two you can turn on Auto-start and the night arms itself when you settle. Then you lock the phone and sleep.
All night, the microphone feeds sound to the phone and only the phone. The listening, the math, and the morning number all happen on the device. Your recordings are never uploaded, and there is no cloud copy of your audio. That design choice has real trade-offs, and we lay them out honestly in your recordings stay on your phone.
Step two: a learned model, kept honest
A night of bedroom sound is messy. There are breaths, silences, snores, a partner turning over, a furnace kicking on, a car outside. To sort that out, Nightsong uses a learned model, the same kind of modern machine learning behind today’s voice and photo tools, trained to recognize the real sound of snoring and breathing.
A model alone is not enough, so we do not let it work alone. Plain, fixed checks run after it and throw out anything that was not loud enough or steady enough to be real. The model is smart. The checks keep it honest. Same night in, same number out.
Step three: two main ingredients
The Index is mostly built from two things a phone on a nightstand can genuinely hear:
- Snore burden. How much of your night had snoring in it.
- Breathing regularity. How steady your breathing sounded, breath after breath.
A smaller share comes from other sleep measures. If you wear an Apple Watch and connect Apple Health, heart rate, HRV, and oxygen can join the morning picture. Nightsong does not measure those with the microphone, and a watch is never required. It enriches the picture. It never gates it.
Step four: you against you
The number is set against your own baseline, never a stranger’s average. That is why the first nights read LEARNING, or show a count like 0 of 7 nights. Nothing is broken. The app is learning what normal sounds like for you. Missed or unheard nights do not count toward the baseline, so keeping the same spot, plugged in, every night is what makes it yours. More on that in what a baseline is.
When the night was only partly heard: Partial
Sometimes the phone hears part of a night well and part of it poorly. Maybe you moved the phone at 2 am, or a fan was switched on beside it. When that happens, the Index is marked Partial. It reflects the stretch of the night that was heard well, and the label tells you so. A Partial night is still useful. It just is not the whole story, and we would rather say that out loud.
When the night was not heard: no number
If the night was not heard well enough, Nightsong gives you no number. You will see a NO READ state, or a plain line saying we did not hear the night, instead of a score. This is deliberate. A made-up number would feel friendlier for one morning and would quietly poison your trend for a month.
The usual causes are simple: the phone ran out of battery, the night never started, the microphone was buried under a pillow, or a fan or noise machine sat right next to the phone. Placement is the whole game.
What the Index is not
The Somnus Index is a wellness number. It describes your own nights against your own baseline. It does not diagnose anything, it does not screen for any condition, and it is not a reason to start, stop, or change any treatment. If you are wondering whether Nightsong is right for you at all, read who Nightsong is not for, and for the short factual version of everything, see Nightsong at a glance.
The beta is full right now. If you want a spot when one opens, join the list.
Questions people ask
What goes into the Somnus Index?
Mostly two things the phone can hear: snore burden, meaning how much of the night had snoring, and breathing regularity, meaning how steady your breathing sounded. A smaller share comes from other sleep measures. All of it is read against your own baseline.
What does a Partial Index mean?
Partial means Nightsong heard only part of the night well. The number reflects the stretch it could hear, and it is labeled so you do not mistake it for a full night.
Why did I get no number this morning?
When the night was not heard well enough, Nightsong shows no number rather than inventing one. The usual causes are a phone that ran out of battery, a night that never started, a covered microphone, or a fan or noise machine right next to the phone.
How long before the Index means something?
The first nights show LEARNING or a count like 0 of 7 nights while your baseline forms. Missed or unheard nights do not count toward it. Keep the same spot and the same routine, and it settles.
Do I need an Apple Watch?
No. The phone does the whole job. If you wear an Apple Watch, its numbers can add context in the morning, but the Index is still mostly snore burden and breathing regularity from the phone.