Bitcoin market is lively 24/7 and if you know it, you will not say it usually drops or rises overnight because it is not enough information. It can be overnight hours in your local time but it is possibly day time for people who live in other nations with different time zones.

You will see a clearer picture if you can connect most volatile hours of Bitcoin belong to which nations in that time zone. If you notice it and also lay attention on news and FUD, you will see the USA has kind of connections to Bitcoin most volatile hours which can be overnight hours in your nation and local time.