We have a web server that we have found (through Microsoft Monitoring) does not seem to be releasing sessions until our app pools recycle at night. The graph is below.
Is there any dT tools that can help us figure out why this is happening?
that's a very general question, so the general answer is: likely yes!
Since this seems to be some kind of leak where the application keeps your sessions referenced and doesn't free them it's a typical usecase that you can use dynaTrace to diagnose. I'm confident you will find the architectural problem and even the code snippet that is responsible for this behavior quite fast.