whole array frequently spinning up when not in use

Title pretty much says it all.

I have 1 x 4TB HD parity, 4 x 4TB HDs, 2 x 500GB SSDs, one of which is currently unused, one is the cache drive for most shares, and 1 x 8TB unassigned device drive, and the flash drive of course.

Sometimes things seem to be operating as I'd expect with the cache and maybe one of the array drives having a green light on the Web UI, because someone in the house is streaming from Plex, the others being grey to indicate they're spun down.

However, for no reason I can fathom, five minutes later, when I check the Web UI main page all the drives are lit up green when nothing else (I'm aware of) has changed. Why the unassigned device should spin up, even when some other automatic process is spinning up the array, is weird.

In the disk settings I have them set to spin down after 15 minutes, but they don't spin down. I've tried resetting this (the changing the setting to 30 min, applying and then changing it back again trick I read about on these forums) and the server was rebooted yesterday.

Is there a setting I'm missing? Maybe even something I need to change in the UEFI BIOS?

Generally very keen to learn about anything I can do to reduce power consumption and running costs.

Thanks