Hello Bond,
About the information you added on your last replies, it is quite interesting that the DB% increased around 21:00. Did you checked on transaction SM37 the jobs scheduled from 21:00 to 22:00 to see if any of them has a high execution time?
On transaction ST03N it is possible to check which processes and users expend more time on CPU, Database, etc. so what you can do is to check which one has the highest Average Response Time on Database and compare it within a few days. Also you can get a ranking list with the top response time per processes. The idea is to get which user and program is raising the Response Time and try to find a solution for it.
Regards.