I have a sata WD hdd (WDC WD500) connected to MSI P35 Platinum motherboard.
It goes to sleep and wakes up instantly. I can hear how it starts (spins out) and after a few seconds it goes sleep (no noise). Then after a few seconds the cycle repeats itself. And it's like this all the time.
In addition to this hdd I have 3ware raid controller (AMCC 9650SE-2LP) with to serial ata disks connected. I think the problem is not related to this controller, though.