can i leave my Apple Watch 8 on charge overnight

The Apple Watch regulates the power it draws in. The wattage of the charger does not matter, as the Apple Watch is in control.

I have a 4 and a half year old Series 4.

It goes on the charger when I go to bed, and stays there all night.

At the end of the day, my Series 4 typically has 40-50% charge remaining.

So putting my Apple Watch on the charger every night, all night long, has not hurt it in anyway.

Absolutely.

The "charger" for a device is built into the device. It is not the thingy that plugs into the wall, and it is not the cable that connects the thingy that plugs into the wall to the phone. They are just a source of current and a way to get it to the phone, respectively. For more info, see:

When to charge your iPhone or iPad - Apple Community

which also applies to the Apple Watch as well.