Garbage collection happens when the SSD is in an idle state not a sleep state. If the SSD is in sleep it will not pull any power. Garbage collection is transparent to the user so I am not sure how you would verify this is taking place at all. Additionally if you have enabled TRIM garbage collection will never really run because TRIM take care of that in real time.
You may have some other issue going on but I do not think it would be related to garbage collection.
