In case that fails please follow these recommendations:
Make sure you have installed the latest version of iOS and watchOS
This ensures that you running the most current version of all software components of that could be responsible for this issue.
Make sure your devices are properly connected with each other
Pillow on the Apple Watch and the iPhone communicate via Bluetooth and/or wifi. If you have enabled Airplane mode or disabled Bluetooth or wifi automatic synchronization will not work. Also, make sure both devices are withing 10m between each other.
Try synchronizing your sleep data manually
Pillow offers the option to synchronize your data manually.
To do that:
1. Make sure both devices are properly connected with each other (as described above).
2. Open Pillow on your Apple Watch
3. Tap on the Sleep analysis icon on the bottom right, to bring up the sleep analysis view
4. 3D Touch anywhere on screen to bring up the option to synchronize
5. Select "Sync session to iPhone"
Restart your devices
Restart your Apple Watch and iPhone and try synchronizing again.
If the problem persists please contact our support team to help you right away.