Apple Watch Series 4 with iOS 10 wakes up when in nightstand mode

I have an Apple Watch Series 4 that has iOS 10.3 on it. It is set to optimized battery and nightstand mode is on. When it’s on the charger at night it still charges to 100 before I get up. When it does, it continually lights up and chimes. This happens between 2-4 in the morning and only way to stop it is to take it off charger. It’s driving my husband crazy. Help please! Why is this happening and how can I stop it?