BODY METRICS
What your body is telling you overnight
July 16th 2026・by Andrej Mihelič
GENTLER 101
FITNESS
HEALTH
Your smartwatch can collect five key metrics while you sleep. Each one reflects a different aspect of your physiology, and together they paint a clear picture of how well you are recovering, and whether something might be off.
Sleeping Heart Rate
Your sleeping heart rate (SHR) is an average heart rate per minute while you are asleep. A lower number generally indicates a healthy and strong heart and good cardiovascular fitness. A declining SHR over time is a sign you are getting fitter. An elevated SHR, on the other hand, can signal fatigue, inadequate recovery, or early signs of illness before you are even aware of them.
Good to Know
You may notice a different sleeping heart rate in our apps than in Apple Health. Our SHR, as in most other health and fitness apps, is an average of all heart rate readings taken while you were asleep, whereas Apple's is usually closer to the lowest reading taken overnight.
The average gives a truer picture of recovery. After late training, your heart rate typically starts high and drops through the night as your body settles. Looking only at the lowest reading can miss that, making two very different nights, one following late training and one without, look almost the same. Apple's approach, on the other hand, is good for tracking when there is something off, like illness, because in that case even your lowest heart rate during sleep will be higher than usual.
Heart Rate Variability
Heart rate variability (HRV) measures the variation in time between consecutive heartbeats. It is a good indicator of how well your body can adapt and respond to stress. High HRV indicates resilience and strong recovery capacity. Low HRV can point to fatigue, dehydration, anxiety, or illness, and may imply a need for rest. HRV is highly individualised and is affected by age, gender, and lifestyle, so what matters most is your personal trend rather than any absolute number.
Both apps, Gentler Streak and The Outsiders, offer two calculation methods. RMSSD is widely recognised as one of the most accurate ways to track the impact of training load and short-term recovery. SDNN is considered the gold standard for medical stratification of cardiac risk, and is also Apple's default method. For most athletes, RMSSD is the preferred method, which is why you may see different overnight HRV values in our apps than in Apple Health.
It is worth knowing that HRV is hard to measure well. It is very sensitive to anything you do, even breathing differently, so it is best measured during sleep, and you need lots of readings to get an accurate overnight value. Because Apple Watch only records a few HRV readings per night, both apps put more focus on sleeping heart rate than HRV.
Gentler Streak focuses on your recovery today and how your latest activities affect it. It's designed for people who prefer simplicity over data obsession, unlike our more data-rich The Outsiders.
Wrist Temperature
Your wrist temperature is measured overnight. Small fluctuations are normal due to factors like diet, exercise, alcohol consumption, menstrual cycles, illness, and sleep environment. A notable and sustained rise above your norm is often one of the earliest signs that your body is fighting something, sometimes before other symptoms appear.
Respiratory Rate
Respiratory rate is the number of times you breathe per minute. It is a general indicator of cardiovascular fitness and overall recovery. Your respiratory rate increases when your body needs more oxygen, which can be caused by a challenging training block, allergies or asthma, smoking, heavy alcohol consumption, illness, lack of sleep, or high altitude. An elevated overnight respiratory rate is often a reliable early warning sign of illness or overreaching, but as with the other metrics, it is the trend that matters.
A low respiratory rate, on the other hand, is usually a sign you are well rested. That said, if your respiratory rate trends low over a longer period, it is worth mentioning to your doctor.
Blood Oxygen
Blood oxygen (SpO₂) represents the percentage of oxygen your red blood cells carry from your lungs to the rest of your body. It indicates how well your lungs, heart, and circulatory system are working. Blood oxygen levels typically range from 95 to 100%. Lower values during sleep, possibly below 95%, are normal and can be expected. However, readings consistently below 90% can be a sign that something is off and are worth paying attention to. Individual values vary.
Analyse your body metrics in depth in The Outsiders app with 7-day, 4-week, and 6-month views of your complete Apple Health history.
Why These Metrics Matter Together
No single metric tells the full story. The value of tracking all five is in the patterns they create together. A mildly elevated sleeping heart rate might be unremarkable on its own. Pair it with a drop in HRV, a slightly raised wrist temperature, and an elevated respiratory rate, and the picture becomes much clearer.
This is exactly how activity guidance in Gentler Streak and Training Readiness in The Outsiders use these metrics: not in isolation, but in combination with your training load ratio and sleep quality. The Outsiders app also looks at your 7-day averages, not just last night's readings. If something has been off over the past week, that accumulated signal still has an effect on your readiness today, even if yesterday alone looked fine.
How I Use it
Sleeping heart rate is the one I track most closely. It is a great metric to watch related to daily recovery, but what I really like is the 6 month view in The Outsiders, where you see monthly averages and can easily spot if SHR is rising, and if it is, it might be time to ask myself: am I training too hard and risking overtraining?
HRV is, according to the research, a great metric for tracking recovery and the impact of training load, but it is easily thrown off by things like movement and breathing, so you need lots of overnight readings averaged together for a reliable picture. Not every tracker gives you that many, which is why sleeping heart rate is my go to: plenty of readings, and steady even when my breathing shifts.
The rest are support acts. If daily SHR is up but HRV, wrist temperature, respiratory rate, and blood oxygen all look fine, I do not worry. If two or three are off together, I listen.
Andrej Mihelič, co-founder at Gentler Stories LLC
Frequently Asked Questions
What is a good sleeping heart rate?
A lower sleeping heart rate generally indicates a healthy, strong heart and good cardiovascular fitness. Everyone's baseline is different, so a declining trend over time matters more than any single number.
How reliable is HRV for tracking recovery?
HRV is a good indicator of how well your body can adapt and respond to stress, but it is hard to measure well. It is very sensitive to anything you do, even breathing differently, so it is best measured during sleep, and you need lots of readings to get an accurate overnight value. Because Apple Watch only records a few HRV readings per night, both apps put more focus on sleeping heart rate than HRV.
Should I use RMSSD or SDNN for HRV?
For most athletes, RMSSD is the preferred method, as it is widely recognised as one of the most accurate ways to track training load and short-term recovery. SDNN is the gold standard for medical stratification of cardiac risk, and is also Apple's default method.
How to read body metrics to make the most of them?
It is best to read them together rather than in isolation. A mildly elevated sleeping heart rate might be unremarkable on its own, but paired with a drop in HRV, a raised wrist temperature, and an elevated respiratory rate, the picture becomes much clearer. Focus on your personal trend rather than any single night's reading.
