If a cell is already running ( I assume it's a streaming operation), then I think it doesn't mean that the cluster is inactive. The cluster should be running if a cell is running on it.

On the other hand, if you want to keep running your clusters for a specific period of time (say 10pm to 8am), then you can schedule a cron job to invoke at 55th minute to run a basic command in a simple notebook on that very cluster. You can schedule the job execution using a cron expression.

Also, using cluster API, you can monitor if the cluster is running or not.