A 0-100 summary of the night, combining how long you slept, how much of it was deep and REM, and how often you woke. 80 and above is a strong night, 60-79 is adequate, below 60 means the night is unlikely to have restored you. It is a summary, not a diagnosis - a single low night matters far less than a run of them.
Light, deep and REM sleep serve different purposes. Deep sleep is when most physical repair happens; REM is associated with learning and mood. A watch infers stages from movement and heart rate rather than measuring brain activity, so treat the split as an approximation and watch the trend rather than any one night.
The gap between the sleep you have had recently and the sleep you appear to need, accumulated over several nights. It is why one long night does not fully undo a hard week - the shortfall builds up faster than it clears.
How close your bedtime and wake time are from night to night. Consistency matters independently of duration: going to bed at wildly different hours makes the same total sleep less restorative, because your body clock never settles.
Your heart rate at complete rest, usually taken as the lowest sustained value overnight. It tends to drift down as fitness improves and up when you are tired, ill or under stress. The useful signal is a change from your own normal, not the absolute number.
The variation in time between consecutive heartbeats. Higher variability generally indicates a well-recovered nervous system. It is highly individual, so compare against your own baseline rather than anyone else's.
Overnight blood oxygen saturation and breaths per minute. A wrist sensor is not a medical device: readings carry real error, especially with poor fit or cold hands. Use them to notice a change in your own pattern, and take any persistent unusual reading to a professional rather than self-diagnosing.
A device estimate of physiological strain, derived mostly from heart rate variability. It reflects total load on the body - training, illness, poor sleep and life stress all raise it. It cannot tell those causes apart.
How far your skin temperature overnight sat above or below your own usual, in degrees. Read it as a change from your baseline, not as an absolute number: a single night off says little, while a rise that holds for several nights can show up alongside tiredness, illness or a hard training block. A wrist sensor measures the skin, not your core body temperature, and room temperature and bedding move it too, so it is a prompt to look at how you feel and keep the day easier, not a medical reading. If it stays unusual alongside symptoms, talk to a professional.
The number of steps your watch or phone counted today. It shows how much you moved outside your recorded workouts: a day of long sitting and a day on your feet recover differently, even with the same session. Counts differ between devices and miss steps when you are not wearing the watch or carrying the phone, so compare with your own usual days rather than a fixed target. Steps are informational and never change your readiness or verdict on their own.