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.
Your sleeping heart rate (SHR) is the number of times your heart beats per minute while 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.
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.
The app offers 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. For most athletes, RMSSD is the preferred method.
Wrist temperature is measured while you sleep. Everyone's baseline is different, and 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 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.
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.
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 Training Readiness uses these metrics: not in isolation, but in combination with your training load and sleep quality. It 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.