Absolutely OK to leave it on charge overnight - I always do. The iPad has, as the previous posters have suggested, a sophisticated Power Management system that closely monitors the battery voltage and current consumed by the iPad as well as the charging current. It calculates the optimum current with which to charge the iPad's Lithium Ion batteries, adjusts this dynamically and, when the battery is fully charged, switches the charging current off or into 'trickle charge' mode. It also is responsible for the little '%' battery symbol that you see on the iPad's top menu bar. It also monitors the battery temperature because the optimal/allowable current is dependent on the temperature of the battery too. If the battery should overheat, then the charging current is switched off.
You may recall that Apple had some battery 'issues' a few years ago and a lot of effort went into ensuring that that didn't happen again!! Once bitten, twice shy!!
Tim
Scotland