As far as I can tell there is no direct option for smart wake up with Withings Sleep. This option is available only for Withings Aura or wearable trackers.
I asked about that in review but in answer I received dead link to support topic. Is there any smart alarm function or are there plans for implementing it for Sleep similarly as other trackers have? With this option it would be best affordable tracker on market but if you want to use smart wake-up function not wearing anything only older Aura supports it.