What your Garmin knows about your nights — and how SAYA reads it.

1 · Your nervous system, measured

Every evening, your body decides what kind of night this will be: a night of repair — or a night of vigilance. That decision is made by the autonomic nervous system (ANS), the involuntary control circuit with two branches: the sympathetic branch (fight-or-flight: mobilize, accelerate, stay alert) and the parasympathetic branch (rest-and-digest: repair, refuel, recover).

You can't feel which branch is winning. But your wearable can measure it — through two signals:

Heart rate (HR) is the throttle. When the sympathetic branch dominates, your heart beats faster — even at rest, even in bed.

Heart rate variability (HRV) is subtler and more interesting: it's the tiny variation in time between consecutive heartbeats. A heart under parasympathetic control doesn't tick like a metronome — it breathes with you, speeding up and slowing down flexibly. High HRV means your recovery branch is in charge. Suppressed HRV means your body is still braced for action.

Garmin watches record both around the clock. SAYA's job is to read these signals at the moments that matter most — the evening and the first hours of sleep — and turn them into decisions you can act on tomorrow evening.

2 · The evening resting heart rate — tonight's forecast

Most apps look at your resting heart rate in the morning, summarizing a night that's already over. SAYA adds a measurement that looks forward: the evening resting heart rate (eRHR) — how low your heart rate settles in the final hour before sleep.

Why it matters: an elevated heart rate during the night is linked to less restorative sleep and, in long-term studies, to worse health outcomes [1], and your body's pre-sleep state predicts the night remarkably well — in one 2025 study of national-level athletes, pre-sleep heart-rate-variability measures distinguished chronic insomnia with striking accuracy and predicted measures of sleep continuity [2].

How SAYA measures it: from 70 minutes to 10 minutes before your usual sleep onset, SAYA takes your Garmin's heart-rate stream, applies a 5-minute rolling average, and records the lowest settled value. No button to press — it happens automatically, every evening. The value is then compared with your personal 7-day baseline, not a population table.

The best part: unlike your morning RHR, the evening value is actionable the same evening. A single session of slow, paced breathing measurably raises vagal (parasympathetic) activity [3] — so a "running hot" reading isn't a verdict, it's a prompt. More detail: evening resting heart rate and sleep recovery.

3 · The HRV-Incline — the stress you take to bed

Here is a pattern almost nobody looks at, and it's SAYA's signature metric.

During sleep, the recovery branch gains the upper hand: parasympathetic activity rises as NREM sleep deepens, and heart rate falls across the night [4]. Deep sleep is concentrated in the first part of the night — so on a calm night, HRV climbs early and then holds a plateau. That climb is recovery happening in real time.

The question is: where does the climb start?

If you go to bed calm, your HRV starts near its plateau — little climbing needed. If you carry stress into bed — a late interval session, a heated email, two glasses of wine — your HRV starts deep below the plateau. Your body spends the first hours of the night climbing out of a hole instead of repairing. We call the size of that hole the HRV-Incline: SAYA fits the initial slope of your night's HRV curve, finds the plateau, and computes the area of the deficit between them.

A small climb is normal and healthy. A large one is a fingerprint of pre-sleep stress: late intense exercise measurably suppresses early-night HRV and elevates heart rate [7], alcohol does the same dose-dependently [5], and stress research links it to more fragmented sleep [9]. The full explainer — thresholds, examples and limits — is on the HRV-Incline page.

4 · Chronotypes — measured, not guessed

Your chronotype is your body clock's preferred timing — when your biology wants to sleep, eat and perform. Fighting it has a name, social jetlag, and a cost: sleeping against your chronotype is associated with worse subjective sleep quality [11].

Most apps determine chronotype with a questionnaire. Questionnaires are decent — the short rMEQ correlates strongly with the full research instrument [10] — but they measure what you think about mornings. Your watch can measure what your body does.

How SAYA does it: chronobiology's standard yardstick is your mid-sleep point — the clock-time halfway through your sleep, corrected for sleep debt (the MCTQ method pioneered by Till Roenneberg's group). SAYA computes it from your actual recorded nights and places you on a seven-type spectrum from extreme early to extreme late — with an explicit confidence level that grows with your data (learning < 7 nights, high ≥ 14).

Once measured, the chronotype anchors everything else: your recommended bedtime window, your wake profile, and the timing advice below.

5 · The evening levers — what actually moves the needle

SAYA tracks your evening because the evening is where the research says the leverage is:

Alcohol is the bluntest lever. It sedates you to sleep, then keeps your heart rate up and your HRV down — and the more you drink, the longer it lasts. In a controlled laboratory study, heart rate during sleep was about 4% higher after a low dose and about 14% higher after a high dose, with HRV suppressed, most strongly early in the night [5].

Late, heavy meals can disrupt your sleep: eating within ~3 hours of bed is associated with more night-time awakenings, and hard-to-digest meals make it worse [6]. Digestion competes with recovery — SAYA scores your dinner's timing, size and processing level for exactly this reason.

Late intense training is the athlete's classic own-goal. Finishing hard sessions within a few hours of bedtime delays sleep onset, dampens night HRV and raises night heart rate [7] — while the same session earlier in the day is harmless or beneficial. SAYA learns your personal cutoff from your own nights.

Caffeine lingers longer than it feels: even coffee six hours before bed measurably cost sleep in controlled research [8] — so an afternoon espresso may still tax your night.

Stress doesn't just delay sleep — it fragments it. Stress research consistently links it to more wakefulness after sleep onset — one plausible reason for the "slept 8 hours, woke up tired" feeling [9]. It's also the single strongest pattern in our example athlete's data: evening stress correlated −0.59 with next-morning recovery across 56 nights.

6 · From data to decisions

Everything above condenses into a small set of numbers each morning:

Recovery Score (0–100). A weighted blend of four signals: your Garmin sleep score (10%), an HRV block (60%) that compares your overnight HRV plateau with your personal HRV target and deducts points for a large HRV-Incline, your evening resting heart rate versus baseline (10%), and your morning resting heart rate versus baseline (20%). If an optional signal is missing, the weights redistribute and the score is marked provisional — and if the required data isn't there, SAYA says so rather than inventing a number. Every formula, threshold and limitation is documented in How SAYA analyzes your sleep & recovery data.

Personal baselines, not population tables. Every comparison is against your own rolling history — 7 days for heart rate, 30 nights for your HRV target — cleaned of outliers. A 44 bpm evening heart rate is exceptional for one person and alarming for another; only your baseline knows the difference.

Bayesian timing ideals. Your ideal dinner gap and workout cutoff start from research-based priors (about 3.5 and 3 hours before bed) and shift toward what your best nights show — weighting nights with strong sleep and a small HRV-Incline more heavily. Few data points? The prior dominates and SAYA tells you it's still learning. Lots of data? Your pattern wins.

Honest confidence. Chronotype, ideal duration and timings all carry explicit confidence levels based on how many valid nights back them. No fake precision.

7 · Recovery, readiness — and what a score can (and can't) tell you

Your Garmin already hands you numbers about recovery: Body Battery, HRV Status, a morning Sleep Score, and on newer watches a Training Readiness score. They're good signals — but each is a closed box: it tells you where you landed, not why, and never connects this morning's number to what you did last night.

That "why" is where a recovery score earns its keep, and the physiology it leans on is well established:

HRV is the single most validated marker of recovery. In endurance-athlete research it's the go-to, non-invasive readout of how well the parasympathetic system has rebounded from load [12] — which is why SAYA weights it most heavily.

But a score is only honest against your own baseline. HRV varies enormously between people; the same absolute number can mean "fully recovered" for one athlete and "still fatigued" for another. The research is blunt about it: HRV must be tracked as an individual, longitudinal trend — your own rolling baseline — not against a population table [13]. Every SAYA comparison works this way.

And a single night is noise; the trend is signal. Resting and recovery heart-rate measures are most useful as multi-day trends, not one-off readings [12]. SAYA cleans outliers and reads the direction of travel before it tells you anything.

SAYA is a wellness product, not a medical device. It doesn't diagnose or treat sleep disorders — if you suspect one, please talk to a professional.

Sources

1. Strüven et al., 2025, Nutrients — alcohol, nocturnal resting heart rate and sleep, with background on night-time heart rate and mortality. PMC12073130
2. Li et al., 2025, Frontiers in Physiology — pre-sleep HRV predicts chronic insomnia and sleep continuity in national-level athletes. fphys.2025.1627287
3. You et al., 2021, Int. J. Environ. Res. Public Health — a single slow-paced breathing session raises cardiac vagal activity. PMC8656666
4. Trinder et al., 2001, Journal of Sleep Research — autonomic activity during human sleep as a function of time and sleep stage. doi:10.1046/j.1365-2869.2001.00263.x
5. de Zambotti et al., 2020, Sleep — evening alcohol and nocturnal autonomic function, a dose–response laboratory study. PMC7819834
6. Chung et al., 2020, Int. J. Environ. Res. Public Health — eating within 3 hours of bedtime and nocturnal awakenings. PMC7215804
7. Leota et al., 2025, Nature Communications — dose-response relationship between evening exercise and sleep. s41467-025-58271-x
8. Drake et al., 2013, Journal of Clinical Sleep Medicine — caffeine taken 6 hours before bed still disrupts sleep. PMC3805807
9. Grafe et al., 2023, Neurobiology of Stress — stress and sleep fragmentation (review of preclinical studies). PMC10709081
10. Hwang et al., 2024, Journal of Korean Medical Science — validity and reliability of the reduced Morningness–Eveningness Questionnaire (rMEQ). PMC11458378
11. Raman et al., 2019, Clocks & Sleep — chronotype, social jetlag and subjective sleep quality. PMC7445814
12. Buchheit, 2014, Frontiers in Physiology — resting, exercise and recovery heart-rate measures for monitoring training status. fphys.2014.00073
13. Plews et al., 2013, Sports Medicine — training adaptation and HRV in elite endurance athletes; monitoring needs an individual baseline and longitudinal tracking. PMID 23852425

In this topic

• Evening resting heart rate and sleep recovery
• HRV-Incline: understanding overnight HRV recovery
• Personal sleep analytics: find what actually affects your sleep
• How SAYA analyzes your sleep & recovery data