NOYU shows two heart rate numbers whose names sound interchangeable. They measure different windows of your day, and both become more useful once you can tell them apart.
The two numbers
Resting heart rate is the figure your device calculates for the whole calendar day. It's the same number you'll see in your Garmin, Oura or Apple Health app, and it's the one to compare against the general guidance you'll read elsewhere.
Sleeping heart rate is your average heart rate across the night, measured only during the sleep your device recorded. Nothing from your waking day goes into it.
These are two separate measurements, not a rounding difference. Sleeping heart rate usually reads a little higher than resting heart rate, because it averages across the whole night including REM sleep, when your heart rate rises.
You'll also see lowest sleeping heart rate on some surfaces, which is the slowest your heart beat at any point in the night. That one tends to sit below both of the others.
Why we use sleeping heart rate for Readiness
Readiness describes the state you woke up in: how ready your body is for the day ahead, set from the night that just ended.
Resting heart rate covers the calendar day being scored, so it's still accumulating while you read it. A stressful morning or a hard lunchtime session would feed into the same day's Readiness, telling you about your afternoon rather than about the night your body spent recovering.
Readiness reads your sleeping heart rate instead, compared against your own recent baseline. That gives a cleaner answer to the question the score is asking.
If your device doesn't record heart rate during sleep at all, Readiness falls back to your daily resting heart rate. When that happens the label changes to match, so you can always tell which measurement you're looking at.
Which should you pay attention to?
For most purposes, resting heart rate is the one to watch. It's the standard figure, it's comparable to what you'll read elsewhere, and a steady fall over weeks and months is one of the clearest signs your aerobic fitness is improving.
Sleeping heart rate works as a night-to-night signal. Alcohol the evening before, a late or heavy meal, fighting something off or a hard training day will each lift it. Read a single high night as noise, and a sustained rise across a week as something worth looking into.
How we compare them
We score neither number against a population target. We compare both to your own rolling baseline, because the question worth answering is whether the number is normal for you.
Your baseline is a 30-day median built from the days before the one being scored, so today's reading can never influence the baseline it's judged against. We also account for how much your own numbers bounce around: if your heart rate holds steady, a 4 bpm rise means something, and if it swings about, the same 4 bpm is a blip. You get credit for your own spread instead of a fixed percentage.
A note on wearing your watch overnight
Resting heart rate is reliable when you wear your device to sleep. On days you wear it while awake only, most devices estimate it from your calmest daytime moments, which reads high.
If your resting heart rate looks elevated for a day or two, check whether you wore the device those nights.