Overnight interest is an invisible bill that many traders notice for the first time after holding a position overnight. It is charged on a daily basis, has different long and short directions, and will double on a specific day. This cost is essentially the time value of funds related to the market interest rate environment, rather than a fixed fee arbitrarily set by the platform. This article explains its pricing logic, direction differences and doubling rules, and how long-term position holders can calculate a complete position cost account through cost calculation, direction comparison and interest rate tracking, so as to avoid losing money overnight after saving on spreads.