Hi, JacoMey the smartwatch uses the heart rate sensor, on the back of your smartwatch, to record your sleep cycle.
Clean the back of your smartwatch with a clean cloth. Put the watch back on making sure it's in the correct position.
Change the heart rate settings to 'Always' on.
1. Press the Home button.
2. Select Samsung Health.
3. Select your bpm.
4. Select the three dots.
5. Select Settings.
Wear the smartwatch snugly on your wrist. Don't fasten it too tightly.
Kindly be advised that there may be a fault with your smartwatch. Please go to the below link to locate the closest Authorized Service Centre to you.