My Manager keeps asking me to work complete 8 hours a day

We work in a 5pm-2am shifts. This was not an issue earlier. We were only monitored on the basis on targets. If target has been reached then all well and good but now since few days my Manager keeps pressuring especially me (not sure what I did) that company is paying us for working 8 hours a day and we need to utilise all of it. You can't get your work done early and stay idle for rest of day. Fyi I don't stay idle, I either keep learning something new in that time. I feel a bit targetted and under appreciated because what I feel is if I am having less work that should mean I am staying upto date according to me . Please correct me if I am wrong.

Now I don't have a backup job offer but I am still considering resigning and looking for a new job during the 2 month notice period.

Any suggestions?