On an idle server the change number indexer thread wakes up regularly to check the changelog head.
This can be observed with strace -p <change number indexer thread>
Attaching a debugger it seems that it's being woken up when heartbeat messages are received. In a large topology this could be noticeable number of times per second and leads to a small constant usage of resources.
This is not a significant concern, since I believe as load increases, wake-ups due to heartbeat messages should become irrelevant; the indexer will be active anyway due to real changes coming in.