What is my phone doing at 1 o'clock in the morning? I have recently installed JuiceDefender (JD) and along with it JuicePlotter (JP). JP gives me a nice graph about my energy consumption over time. I noticed that around 1 A.M. my phone uses up more battery power than the rest of the night. I don't think that I have something explicitly scheduled at that time. I have Samsungs 2.2 Froyo (rooted) on my Samsung Galaxy-S (I9000).

How can I find out the cause for this?
You can use a logcat viewer, such as LogCat Reader (I've found that one works well), and just scroll back through the logs until the time in question.  

A less precise solution would be to go to Settings > About phone > Battery use and that will tell you what has been using most of your battery.