# Why each signal is watched on its own

> myCocoon watches your sleep, resting heart rate, steps, and mood as four separate things, each against your own usual from the last two weeks. Here is why one bad reading is usually nothing, how the app stays quiet on most days, and why reading everything together is still to come.

URL: https://trymycocoon.com/blog/why-each-signal-is-watched-on-its-own/
Category: Building myCocoon
Last updated: 2026-09-18

## Why one bad reading tells you almost nothing

Last reviewed: 18 September 2026.

Your resting heart rate was high on Thursday. Your sleep was short on Tuesday. Your step count spiked on Sunday. Any one of these on its own is probably nothing. The question worth asking is not whether a number crossed a line, but whether a pattern is forming against your own usual, not a chart made for everyone.

That distinction matters more in pregnancy than at almost any other time. Resting heart rate climbs through the second trimester, often 10 to 20 beats a minute (bpm) above your pre-pregnancy number, and peaks somewhere around the late second trimester. ([Sanghavi and Rutherford, Circulation, 2014](https://www.ahajournals.org/doi/10.1161/circulationaha.114.009029)) A reading that looks high on a chart made for everyone can be entirely ordinary at 32 weeks. A reading that looks fine on the same chart can be high for someone whose heart rate has always sat quietly in the low 60s. Without your own baseline, an app is measuring your body with the wrong ruler.

So when myCocoon shows a quiet card saying your sleep has dropped by 90 minutes a night for the last three nights, one specific rule decided to say that. Not a model weighing everything the app knows about you. One rule, looking at one thing, against one baseline that is yours. This post is about why it works that way, and what would have to be true before it changed. The earlier post on [what it means for an app to listen rather than track](/blog/ambient-intelligence-pregnancy/) covers the thinking. This one is about the rules underneath it.

## How does the app decide whether to say something at all?

Four small rules run every time the app checks, in the background, when you open the app, or after the morning mood prompt. Each rule looks at one thing and answers with a note or with nothing. The app drops anything it already said today and sends what is new. On most days, all four answer with nothing.

Every check is written down on your phone, including the rules that ran and stayed quiet. That record lives in a part of the phone's storage that never syncs to iCloud or to a myCocoon server, and it is what the drawer on the home screen reads from.

Deciding what to say is deliberately simple. It is a short list of rules, checked one after another, with no model weighing everything at once. That plainness is the point. You can read each rule, understand why it speaks up, and know what it misses. The post on [how myCocoon reads Apple Health data](/blog/reading-healthkit-cleverly/) covers how the readings reach the phone in the first place, and why once an hour is the best the background can do.

## What is your personal baseline, and when does it form?

Your baseline is your usual, worked out from your own last 14 days: resting heart rate, hours of sleep, and daily step count. The app writes a short summary of each day, and once 14 exist it takes the middle value for each of the three. Before that, the rules use standard thresholds that apply to everyone.

The middle value is used rather than the average because one odd night should not move your usual. If you had a rough week a fortnight ago, the middle value of 14 days barely notices. An average is too easy to drag around.

The same rule therefore lands differently for different people. Someone whose resting heart rate usually sits in the low 60s will get the heart rate note when a night-time reading climbs to 76 bpm with a rising week, well under the standard line of 100 bpm. Someone who usually sleeps 8.5 hours will get the sleep note when three nights in a row land under 7 hours, well above the standard line of 5. The line moves to match you.

Each baseline is also independent of the others. If you have shared sleep with the app but not heart rate, you get a sleep baseline only. The heart rate rule keeps the standard threshold while the sleep rule uses yours. Sharing less narrows one rule and switches nothing off.

## What does each of the four rules actually watch?

Each rule checks a single condition and sends at most one note per check. Three of them read Apple Health. The fourth reads the mood you log in the app and needs no Health access at all. Here is the full set, with both the standard version for the first fortnight and the personal version after that.

| Signal | First 14 days (standard threshold) | With your baseline | Extra condition |
|---|---|---|---|
| Sleep | All 3 of the last 3 nights under 5 hours | All 3 under your usual minus 1.5 hours | Three nights in a row, never an average |
| Resting heart rate | Above 100 bpm, and rising over the week | Above your usual plus 15 bpm, and rising over the week | Between 10pm and 6am only |
| Steps | Above 8,000 today | Above 1.5 times your usual | Evening only, after 6pm |
| Mood | Two-week mood score dropping by half a point or more, and 5 or more down days in a row | Same rule, no Health data involved | Five days, not three or four |

**Sleep needs all three nights.** One decent night in the middle breaks the pattern and the rule stays silent. The rule carries a written warning to anyone who works on it later: do not weaken this to an average or two out of three. A single short night has too many harmless explanations. Three in a row is where a pattern starts to mean something. Pooled estimates put clinically significant insomnia symptoms above 40 percent of women in the third trimester, ([Sedov et al., BMC Pregnancy and Childbirth, 2021](https://link.springer.com/article/10.1186/s12884-021-03755-z)) so short sleep is common and the rule has to be strict to be useful.

**Heart rate only counts at night.** Daytime heart rate rises for stretching, walking, a long meeting, a moment of worry. None of that tells you anything. The 10pm to 6am window catches your true resting rate, and the rising-week requirement means a single high reading is never enough. Rising means the average of your three most recent readings is more than 10 percent above the average of the three oldest in the last week.

**Steps is a multiple of your usual, not a fixed number.** If your usual day is 4,000 steps, a fixed line at 8,000 would never notice a day you walked twice your normal. One and a half times your usual means "unusually active for you", and the evening window stops it going off at 11am for a morning walker who has long since sat down.

**Mood waits five days.** The rule compares the average of your first seven mood entries against your last seven over a two-week window. A drop of half a point or more counts as sliding, but the rule also needs five down days in a row before it says anything. That threshold was set during a medical review in April 2026, because three or four flat days sit inside normal mood variation in pregnancy. ACOG's 2023 guideline recommends screening with the Edinburgh questionnaire that midwives use, at a cut-off score of 10. ([Screening and Diagnosis of Mental Health Conditions, ACOG, 2023](https://www.acog.org/clinical/clinical-guidance/clinical-practice-guideline/articles/2023/06/screening-and-diagnosis-of-mental-health-conditions-during-pregnancy-and-postpartum)) The five-day rule is not that questionnaire. It is a check-in that arrives before you need one, and when it arrives it asks whether you would like to check in, not whether you are depressed.

## How does the app avoid saying something too often?

Each kind of note has its own limit per day and its own gap between sends. Sleep notes, rest suggestions, and wellness checks are limited to one a day. Heart rate notes get two. Each also has an eight-hour gap, so the app will not repeat itself before you can act. Quiet hours, 10pm to 7am, hold everything back.

| Note | Most per day | Gap between sends |
|---|---|---|
| Sleep note | 1 | 8 hours |
| Heart rate note | 2 | 8 hours |
| Rest suggestion | 1 | 8 hours |
| Wellness check | 1 | 8 hours |
| Kick reminder | skips the limits | skips the limits |

Kick reminders and safety alerts are the exception. They skip the daily limits, the gaps, and quiet hours entirely, because reduced fetal movement is the one thing the app should never hold back for the sake of a tidy notification count. The app aims for no more than four kick reminders a day, but nothing will ever stop one from going out.

There is one more ceiling that is not the app's to set. iOS hands over new Health readings in the background about once an hour at best for most types, ([Apple developer documentation on background delivery frequency](https://developer.apple.com/documentation/HealthKit/HKHealthStore/enableBackgroundDelivery(for:frequency:withCompletion:))) so the rules often have nothing new to look at. Across a whole pregnancy, each note tends to mean something when it arrives.

When nothing is sent, the drawer on the home screen shows what was looked at and what stayed quiet. If your sleep was a little short but not short enough to cross your own line, that shows as considered, not sent. If a note was held back, the drawer says why in plain words, such as "held back because it was quiet hours". You can see the app looked and chose not to speak, rather than wondering whether it noticed at all.

## Why aren't the signals read as one combined picture?

The honest answer is that it needs more evidence than exists. A combined rule, one that speaks up when sleep is low, heart rate is up, and mood is sliding, needs enough real cases to know whether the combination means more than each signal alone. Those cases are not there yet, so a rule written today would be a guess.

There is a second reason. Each signal is easy to misread in its own way, so each has its own extra condition. Heart rate needs the night window because daytime readings jump around. Sleep needs three nights because one bad night proves nothing. A combined rule would need one condition that handles all of that at once, and it would land either too strict to ever speak up or too loose, going off in a week when one signal dips and another happens to sit right at its line.

I have wanted to build the combined read since early in the project, and I chose to ship the separate rules first because the risk I worried about most was one signal hiding another. If sleep and heart rate only spoke up together, a day when sleep alone was short would be invisible. The sleep rule, on its own, catches it regardless of what heart rate is doing. Combining signals lets the app say more, but it also lets one signal mask another, and I would rather the app say slightly too little in a way you can check for yourself than say something confident with no evidence behind the confidence. How many signals, off by how much, over how many days, add up to something worth naming, and how does that shift by trimester? Those questions need real results from real pregnancies to answer honestly, and we are doing that work.

The app is already built so a combined read can be added later. Right now, each rule decides on its own, then hands the note to the same step that sends it. A combined rule would use that step too, without anything else changing. It ships when it has been tuned against real results well enough to help rather than alarm.

Until then, the daily summary on the home screen gathers all four signals in one place so you can see them together, even though the rules treat them separately. Reading them as a picture is something you and your midwife or GP do. The app's job is to make sure the picture you bring to that conversation is accurate. The full account of where each kind of data lives is on the [privacy page](/privacy/).

## Common questions

**Does myCocoon compare my heart rate to other pregnant women?**

After 14 days of data, no. The app works out your own usual from your Apple Health history and compares recent readings to that. Before 14 days it uses the standard thresholds in the table above. The switch happens without a notification.

**Does a short night and an elevated heart rate together cause a louder alert?**

No. Each signal has its own rule. If two rules speak up on the same day, you see two separate notes, and neither one hides or amplifies the other. A rule that reads the combination is planned but has not shipped.

**How many nudges can myCocoon send in a day?**

One sleep note, two heart rate notes, one rest suggestion, and one wellness check, each with an eight-hour gap between sends. Kick reminders and safety alerts skip the limits and quiet hours entirely.

**When does my personal baseline kick in?**

After 14 daily summaries, roughly 14 days of Apple Health access. Each signal has its own baseline. If you have shared sleep but not heart rate, the sleep rule uses your own usual while the heart rate rule keeps the standard threshold.

**Can I see what the app decided not to show?**

Yes. The drawer on the home screen lists which rules ran today and which stayed quiet, and gives a plain reason when a note was held back.

**Does the personal baseline data get sent anywhere?**

No. Your baseline is worked out from daily summaries kept in a part of the phone's storage that never syncs to iCloud or to a myCocoon server. When Cloud AI is on, a daily summary of recent health signals goes to Google's Gemini with your questions, but the baseline itself stays on the phone.

## Bottom line

myCocoon does not watch your pregnancy as one combined health picture. It watches four signals, each against your own usual from the last 14 days, each on its own rule with its own daily limit. On most days all four are quiet. When one has something to say, it says one sentence, and the drawer shows you why.

The combined read, where the app notices sleep down, heart rate up, and mood sliding, and treats the three as one signal, is planned. The place for it is built. The evidence to support it responsibly is still forming, and shipping it before then would be the wrong kind of confident.

myCocoon is on the App Store for iPhone on iOS 26 and later. It is iPhone only, with no Android version, no peer community feed, and a smaller reference library than the big-name trackers. The privacy design is our own claim, not yet checked by an outside auditor. What ships today is four separate rules, a personal baseline that forms after about two weeks, and limits that keep notes rare.

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