Sleep tracking

Why sleep matters for weight loss

Poor sleep affects hunger hormones, energy levels, and your body's ability to recover from exercise. People who sleep well tend to lose weight more consistently — which is why DayByDay tracks sleep as a first-class health signal.

Sleep stages explained

Each night, your body cycles through four sleep stages. DayByDay tracks them all and shows you a breakdown of time spent in each one.

• ✦Awake — brief moments of wakefulness throughout the night; some is normal, too much can fragment your sleep.
• ✦REM (Rapid Eye Movement) — the dreaming stage; your brain is highly active, and this is where emotional processing and memory consolidation happen.
• ✦Core (Light Sleep) — the most common stage; your body is relaxed but easily woken, and it bridges deeper sleep cycles.
• ✦Deep (Slow-Wave Sleep) — the most restorative stage; your body repairs tissue, builds muscle, and strengthens your immune system here.

The sleep stages donut chart

The donut chart gives you an instant visual of how your night was divided. Tap any segment to see the exact time and percentage for that stage.

There's no single "correct" breakdown — sleep stage ratios vary by age, health, and lifestyle. What you're looking for over time is consistency and an upward trend in deep and REM sleep.

Blood oxygen (SpO2)

SpO2 measures the percentage of oxygen in your blood. Overnight, it should stay between 95% and 100% for most healthy adults. Brief dips below that are common and not usually a concern.

Frequent or sustained drops below 90% can be a sign of sleep-disordered breathing, such as sleep apnea. DayByDay tracks the number of dips per hour and your average overnight level so you can spot patterns over time.

The O2 Score

The O2 Score is DayByDay's overall sleep quality number for the night. It combines your average SpO2, the number of dips per hour, and how stable your oxygen stayed throughout the night into a single 0–100 score.

A higher score means a more stable, oxygenated night. Track it over time to see how lifestyle changes, sleep position, or treatment is affecting your overnight oxygen.

O2 Ring setup (Wellue / Viatom)

The Wellue O2 Ring and Viatom O2 Ring are small finger devices that record your overnight SpO2 and pulse rate in detail — far more granular than most smartwatches. Because these devices use a proprietary binary format, importing data is a quick manual step rather than an automatic sync.

1. 1Wear your Wellue or Viatom O2 Ring while you sleep.
2. 2In the morning, open your O2 Ring's companion app (ViHealth or O2Ring) and sync the device via Bluetooth.
3. 3In the app, find the overnight recording and tap Share or Export to save the data file.
4. 4Open DayByDay → Sleep tab and tap Import.
5. 5Select the file you just exported. DayByDay will parse it and add the overnight data to your sleep history.

Apple Health sleep sync

If you have an Apple Watch, or any app that writes sleep data to Apple Health, DayByDay pulls that data in automatically. This includes sleep stages, heart rate, and respiratory rate recorded by watchOS.

To enable this, go to Settings → Health sync and make sure sleep permissions are turned on. Data syncs each time you open the Sleep tab.

Sleep debt tracker

Sleep debt is the gap between how much sleep you actually got and how much your body needed. DayByDay tracks this cumulatively over the past week and shows you how much recovery sleep would help.

You can set your target sleep duration in Settings → Sleep. The default is 8 hours, but many people function best at 7 or 9 — set it to what's right for you.

Overnight vitals

In addition to sleep stages and blood oxygen, DayByDay shows two other overnight signals when they're available: resting heart rate and respiratory rate. Both come from Apple Health or compatible wearables.

Lower resting heart rate over time is generally a sign of improving cardiovascular fitness. Respiratory rate during sleep can also reflect recovery quality — elevated rates sometimes correlate with illness or high stress.