If you have Apple's FileVault encryption activated on your computer and it slips into standby, it will automatically save the encryption keys in the RAM so that the system can quickly resume without having to unlock the entire volume again. Although this may seem like a convenient feature, you might want to stop this from happening in order to ensure maximum security for your system.
To disable the systems ability to store the keys, you need to make a small change to a setting in the system management controller (SMC), which can be done by running the following command in the OS X Terminal program:
sudo pmset -a destroyfvkeyonstandby 1
Once you have run this, it will ask you for your password, pop it in and then reboot your machine and the system will no longer store the FileVault encryption keys in memory when it enters standby.
If for some reason you nee to undo this change then simply repeat the command but switch the 1 for a 0 instead.