Getting crazy with battery discharging / draining overnight

Closing the lid is a REQUEST to put your Mac to sleep.

However, there are many, many things that can keep that sleep from occurring when requested.

To completely side-step this problem, connect the power adapter at night, and your Mac can adjust its battery level in the optimum way under software control. Your Mac will NEVER over-charge.

if you would like to research exactly why it may not be sleeping, readers can help you with that, but it will take some work on your part.

In addition to @Grant's great advice & explanations, you should make sure to first disconnect all external devices including the charging cable prior to sleeping or powering down the laptop since connecting or disconnecting item may wake up or power on the laptop. You also must not touch any keys or even the Trackpad as those will also trigger the laptop to wake up or power on again.

Another thing to consider is whether the laptop battery was actually at 100%. Even if the power adapter is connected all day do not assume it is at 100% since Optimized Charging may leave the battery at 80%.

I completely agree that you should completely power off the laptop.

FYI, if you don't use this laptop very often, then make sure you do not leave this laptop at 0% charge for any extended amount of time or you will permanently damage the battery.

Lid-down is a Request for sleep. It is entirely possible, even common, for that requested sleep to NEVER occur.

if you check the [√] re-open windows when logging back in

checkbox, you previous state will be restored in seconds after powering back up.

it is not a good idea to sleep your Mac for hours while in transit. Many bad things could happen along the way.