In the middle of the night, $BTC crashed into a huge pit. Since there was a lot of margin, I thought setting a stop-loss would be too much of a loss, so I opened a hedging position before going to sleep. Although I didn't close it at the best position to take profit after the huge pit appeared, it's better than losing money. Lesson one: stop-loss or hedge, a must when going to sleep, otherwise you're in big trouble.