It has been very widely reported that Google Fit syncs sleep data with third-party apps. One example is Sleep as Android, which is a low-budget app development, compared to the enormous resources of Withings/Nokia.
I have been stuck in customer support hell with a long round of going-nowhere messages from Withings, completely avoiding this subject. So, here I am.
The whole point of the Withings Sleep device is to aggregate valuable data. And the marketing materials for Withings Sleep absolutely lay claim to syncing with Google Fit. It simply does not work, and when you ask Withings why, they dodge the issue, pretending to have no clue what we're even asking about.
I hope that they'll finally take this seriously, and fix this long-lasting bug without turning it into a gigantic delayed project. The API clearly already facilitates this, and all Withings needs to do, is respect their customers and get moving. It's even a question of fair trade regulation: they literally claim to offer a product feature that hasn't worked from the beginning.