The alarm does not go off when my iPhone is plugged into a charger

The Smart Sleep Alarm will go off when the iPhone or iPod touch are plugged into a charger.

But we have found some 3rd party USB iPhone cables which block the iPhone from making sounds. If you charge your iPhone while recording sleep, please ensure you are using the Apple branded USB cable that came with your iPhone or iPod Touch. If you use a 3rd party cable, check that your iPhone can still make sounds when it is plugged into power with that cable.