Oracle SQL在一定时间范围内使用sysdate

I'm trying to select data from the previous day, and within a certain time frame, but I may be calculating my where clause incorrectly. I've tried switching times around etc. Basically I want to see all data from 6am-6pm, and then 7pm-3am, but My results aren't relecting such. I've tried between trunc(sysdate)-1 '00:00:00'<- but specifying the time, but I feel I'm not familiar enough with the function.I'm trying to select data from the previous day