I need some mechanism that allows me to send the raspberry pi as well as the hdd's to sleep
You cannot. Pi's do not have any form of power management or sleep capabilities. You might, if you want to compile your own kernel, be able to get them to hiberate to a swap partition, but I have not tried this and am not promising it will work. Unless you are maintaining a customized kernel for other reasons, it does not seem a worthwhile pursuit since you still need the same physical mechanism for restarting or resetting them as you would if you just shut them down.
You can use the RUN pins (on B+/2 models these are by the DSI cable connector, on the 3 between the 40-pin breakout and the USB jacks) to reset the Pi after it has been shutdown but:
If you want to do that remotely, you'll need a whole other gadget powered on and waiting to receive the signal to do so.
While they use less power once the operating system has been shutdown, they still use power, so if this is about power, it is not ideal.
You could mitigate #2 by using a separate gadget that controls the power supply, so you could shut a pi down, then kill the power, turn the power back on to reboot.