Bitcoin price pumps and dips anytime of the day and not only overnight. I feel that your timezone is why you feel that it's always in the middle hours of the night. However, volatility is the nature of cryptocurrency and fundamental analysis and investors sentiment can come up anytime and thay will affect the market.
I don't think this is what the OP probably meant. This is a common expression in some countries. "Overnight" means very quickly or suddenly, and it doesn't mean it only happens over night.
In any case, this is very common for Bitcoin and other cryptocurrencies, where volatility is the dominant feature. It's quite normal to see multiple price spikes and falls within the same day or even within hours.