If the problem persists, please try the following:
1) Open the Settings app on your iPhone
2) Tap the "Privacy" menu and select "Motion & Fitness"
3) Flip off and then back on the switch that corresponds to Pillow
4) Attempt to open the sleep statistics on Pillow on the Apple Watch.
Unfortunately, this proves to be an issue not easily reproducible, but we are doing our best to iron-out this problem the soonest possible. Thanks a lot for your patience and feedback.