Heart-rate variability, resting heart rate, and your personal baseline
Heart-rate and breathing readings can change from day to day. Underbark uses them only as context alongside your recent records.
Two readings with different meanings
Heart-rate variability, or HRV, is the variation in timing between heartbeats. Resting heart rate describes how often the heart beats per minute when you are at rest. Apple Health generally reports it as a daily figure drawn from readings taken while you were still, rather than from one measurement you take yourself, so it may not match a number you see on the watch face at any given moment.
They are different readings, and neither has one universally good value for every person.
Breathing rate as overnight context
Where a device records it, breathing rate is another overnight reading that tends to sit in a range that is usual for you. Like heart-rate readings, it is more informative next to your own recent records than against a general figure, and a single night says little on its own.
Underbark shows what was recorded and compares it with your recent history. It does not treat a breathing reading as a finding about your lungs, your airway, or your health.
Why one reading is not the trend
Both readings can vary from day to day. One unusually high or low reading may reflect ordinary variation, measurement timing, breathing rate, or how the record was captured. A pattern across comparable readings can offer more context than an isolated number, but it still requires cautious interpretation.
Everyday circumstances can move these readings too. Illness, alcohol, a late or heavy meal, a warm room, travel, stress, and a hard session the day before are all commonly associated with a shift. Seeing one of these alongside a changed reading does not establish that it was the cause, and Underbark does not attribute a change to any of them.
Your own recent history before population targets
Underbark compares a current record with what is typical for you based on your recent history. This is your personal baseline. It does not assume a population target is right for you. A useful comparison also depends on enough earlier readings being available.
For heart rate specifically, the current app takes its recovery context from the heart rate recorded while you were asleep rather than from the daily resting figure Apple Health reports. The two usually move together, but they are recorded differently, so the resting number you see in Apple Health is not the one being compared when a recovery result changes.
Availability and measurement context matter
The device or app, measurement timing, permissions, device wear, and how much of the night was recorded can affect what is available. Underbark treats a missing or unsupported measurement as unknown and does not substitute a zero.
Use patterns without inventing causes
Two changes appearing together can be associated without one causing the other. A trend can prompt reflection, but the data alone may not explain why it changed.
If a reading concerns you or you need help deciding what is safe, seek advice from a qualified health professional. For help understanding Underbark's behaviour, visit the support page.