Is there a way to find, what is keeping my iMac from going to sleep mode?

Hello,

Recently, my iMac has been hanging not going to sleep by itself. When I stop working, I manually put it to sleep. One day, at ±3H00AM, I realized that the screen was awake and was not going to sleep automaticaly.

My iMac is set to sleep after inactivity. Is there a way to see what are the processes keeping my iMac from going to sleep ?

Thanks in advance.

[Re-Titled by Moderator]