How accurate is a wearable during sleep?

Reviewing and comparing sleep data

Sleep monitoring is a popular feature of many smartwatches and wearables. Devices like Fitbit, Withings, Apple Watch and Biostrap analyse biometrics during your sleep. These wearables are worn on the wrist and use optical sensors to capture your heart rate. I wondered how the optical sensors would compare to a high resolution chest strap HR-monitor.