John McCormack shows how to save the output from sp_whoisactive for later research:
When I took over a fairly troublesome SQL Server, I was always being asked what was running when the SQL Server was slow. That was fine when it was during the day and I could look. But often I would be asked, why was the server slow at 9pm last Friday? Or some other time. It’s not always the easiest thing to do as SQL Server doesn’t natively keep this information for you.
A quick but effective way to automate this is to download and schedule Adam Machanic’s sp_whoisactive stored procedure. I run this every 10 minutes but I have seen it scheduled as often as every 30-60 seconds. It depends on your system and your needs.
This is in place where I work, and I’ve put it into place when consulting for companies without the budget for fancy tools. I really appreciate that Adam Machanic made it a simple option to insert results into a table. Also, John has a step to delete older data, which is critical.