I'd check to see what's keeping the device awake using BetterBatteryStats or Wakelock Detector. Either will do. Could be kernel related, could be app-related. Either way there's fixes for both but you need to know what it is first.
Someone else might be able to suggest better ways of doing this... but from the top of my head:
If app-related - you can either try disabling specific app autostarts using a variety of apps (such as System Tuner Pro... but I think MIUI has it baked in if I remember correctly - Settings > Permissions > Autostart) or alternatively, cap them using Amplify or NoWakeLock (one or other; both need the Xposed Framework). The latter option is needed if it's something that NEEDS to run but is holding a wakelock - i.e. a system component - you shouldn't disable it's autostart but you can set it to be killed in Amplify if it's hanging, keeping the device awake.
If it's kernel related, sometimes changing settings can help (i.e. double tap to wake is one that holds a wakelock often, turning it off fixes that) or in the worst case scenario, using terminal to outright disable the kernel wakelock function (usually eventX wakelock).
Find out what the wakelock is first, then go from there.
