Yes! PulseOn device can be used to monitor HR at sleep.You can learn your average HR during sleep and see the HR graph.
Please note that the battery time is a restricting factor. In normal mode when the device screen is always on, you can constantly record around 4 h 30 min. In settings you can choose the device screen to be in a power save mode which doubles the battery time to 9 hours of recording with a full battery.