When you put in sleep you MacBook using High Sierra the computer get's hot sometimes.
Solution 1: Chrome is preventing sleep
There could be some app like chrome that is making your Mac awake. Open activity monitor and check for the "Energy" tab:
Take a look to a previous screen, as you can see the Chrome process prevents sleep. Kill chrome and let you mac sleep in a pleseant way :)
Solution 2: Network activity
If you are sharing with iTunes, photo sharing or Back My Mac this could be the cause of the awake mac.
Solution 3: Disable bluetooth device waking
Apple menu > System Preferences, then click Bluetooth. Click the Advanced Button, then deselect "Allow Bluetooth devices to wake this computer."
Solution 4: Last recourse
As always try this two steps:
- Reset the SMC.
- Reset NVRAM.
Additionally you can disconnect all connected devices to the computer.
Try to use another user account is als recommended to find the cause of this problem.
macOS in recovery mode has automatic sleep disabled as long the computer is connected to AC.