source: SATA Power Management: It’s Good to Be Green (April 08, 2009)
The ATA8-ACS standard describes four modes of power consumption for SATA products:
Active
The device is fully powered up and ready to send/receive data.
Idle
The device is capable of responding to commands but the device may take longer to complete commands than when in the Active mode. Power consumption of the device in this state is lower than that of Active mode. If a hard drive is present, it is spun up.
Standby
The device is capable of responding to commands but the device may take longer (up to 30 seconds) to complete commands than in the Idle mode. Power consumption is reduced from that of Idle mode. If a hard drive is present, it is spun down.
Sleep
This is the lowest power mode. The device interface is typically inactive and, if a hard drive is present, the drive is spun down. The device will exit the Sleep mode only after receiving a reset. Wake up time can be as long as 30 seconds.