Yes, you can use Pillow on your Apple Watch without your iPhone for most sleep tracking features.
Once Pillow is installed on your Apple Watch, you can start and complete sleep sessions directly from the Watch app. Your sleep data will automatically sync with your iPhone the next time both devices are connected.
However, Pillow will require a connection to your iPhone only if you try to use certain features that depend on it, such as:
Audio recordings
Sleep aid sounds
iPhone speaker or vibration
If you don’t use any of these options, Pillow can run entirely on your Apple Watch, no iPhone needed during the night.