If you just uploaded a bunch of content to the disk, it is busy indexing, thumbnailing, resizing, etc. Depending on the amount of content, it might take a couple of days.
If you want DLNA, you indeed need to have media indexing on, and that is big piece of the above work.
The NAS will wake up regularly to do maintenance tasks along the same lines, check for updates, etc. So many users have reported that even when it goes to sleep, it is only for 15mn or so, your mileage will vary.
The good thing is that this NAS is very silent (no fan, not much vibration compared with a Synology, for example), and NASes are designed to run all the time.