Sleep, measured against what you need.

It looks at your own nights. Your base is the usual length of the nights that were followed by a good heart rate variability reading. Until it has 10 of those, it starts from 8 hours and says so. On top of that it adds up to 45 minutes after a hard day, and a little extra to pay back recent sleep debt.

You set the latest time you want to be up. In a window of up to 30 minutes before it, your Apple Watch watches for a burst of movement or a rise in heart rate, signs that your sleep is lighter, and wakes you with a tap on the wrist. If neither comes, it wakes you at your latest time. It’s a light-sleep sign, not sleep staging. The movement and heart rate it reads stay on the Watch.

How much lower your heart rate is asleep than awake, as a percentage. A dip of about 10 to 20% is typical. A smaller dip than your usual often follows a late meal, alcohol, stress or getting ill. Auralo compares it with your own last 30 nights.

For stages, heart-rate dip and the smart alarm, yes. Auralo also reads sleep from other wearables and apps that write to Apple Health, such as Oura or a sleep app on your iPhone. If two devices record the same night, it picks one so nothing is counted twice.