Brokers get their bid/ask spread oricing by taking the lowest bid/ask price from a sample from many big banks, hedge funds etc. (liquidity providers).
During the night time, many liquidity providers may have closed for business leaving just a few open, causing some kind of temporary night time monopoly leading to higher spreads.
Your broker will only give you the spread it is able to get from the few liquidity providers who are active at night. So, if these liquidity provider increase the bid/ask spread (which they will definitely do because they know other liquidity providers are closed at night) the broker will pass the high spread to you.
