iMac (Late 2012) - Bluetooth switches itself off after overnight sleep

Strange problem, I bought a used and very good condition late 2012 base 27" iMac with no mods.

Sometimes when my Mac sleeps overnight, the keyboard and mouse will not wake the mac up the next day, a press of the power button will wake the screen, but input doesn't work. Restarting the Mac will bring back device input response.

The keyboard and mouse have battery and are double-flashing their lights as if to say they are looking around.

BUT!

I installed TeamViewer to have access to the mac when this happens and found that the reason the input devices don't work is because Bluetooth was fully turned off.

This means that during sleep, the iMac just decides to randomly turn the bluetooth off. Very annoying, because I don't have a spare USB KB & mouse.

https://www.dropbox.com/s/vrwp6v88q226c08/Screenshot%202015-10-15%2009.40.03.png ?dl=0

Using TeamViewer, I tried to 'Turn Bluetooth On' and the button can be clicked but it doesn't change or work. It does nothing. Bluetooth icon on the top right remains grey.

I went to 'Advanced' and I can see that 'Allow bluetooth devices to wake' is enabled.

https://www.dropbox.com/s/k7oc1j5zy9jj06w/Screenshot%202015-10-15%2009.42.20.png ?dl=0

Anyway, I did a PRAM reset now to see if that prevents it happening again. Will report back.