can you provide us with more info on this issue? How many HBAs do you have connected to the host? Are there any IO/throughput-intensive VMs working on that host? Do you observe the same latency on the array itself? Did you check if the disks are busy on the array? This would help us get closer to the root cause of the issue.
The latency during the night could be because of backups running on the array and that is normal.
If you are using RR, then I would say it is odd to only see this on one HBA. I have seen similar behavior (EMX Symmetrix VMAX 40K) where a faulty SAN port caused issues with one specific path. I identified the path by running an iometer workload using a Fixed PSP. With this running I then systematically changed the preferred path to see how this impacted performance. One of my 4 paths performed poorly, while the others were fine. Once the FC was moved onto a new port everything was back to normal (RR performed as expected again).
Perhaps you have something similar?
The SAN hardware vendors should provide guidance on the recommended PSP. They normally provide a "best practice" document with guidance on configuration.
What array are you using?
Looking at this HP XP7 technical white paper, the recommended best practice PSP is RR;