Sleep Cycle has two motion detection modes, microphone or accelerometer.
You can change modes under:
Profile Tab > More > Motion detection
Microphone for iOS
The built-in microphone is used to analyze your movements as you sleep. Keep the app running in the foreground with the device unlocked. It’s important that you place your phone correctly. Note that iPhone 8 and later versions can be placed facing up.
It’s important that you place your phone correctly as shown below:
Accelerometer
The accelerometer is used to analyze your movements as you sleep. Keep the app running in the foreground with the device unlocked. It’s important that you place your phone correctly (as shown below):
Depending on your type of bed and mattress you might find certain placements work better. Please do not place your phone underneath a pillow or thick mattress as it can become warm whilst charging.
If you feel uncomfortable having your phone close to your head, you may set it to Flight Mode. Keep the app running in the foreground with the device unlocked. As a bonus this also ensures that no one can bother you while you sleep.