Nowadays chargers all very smart, can cut off the charging power once it detects the batteries are fully charge, so I don't know if there's a functional advantage to overnight charging, compared to, say, the old days of trickle charging NiCds, which may take more than 10hrs.
Once a battery is fully charged, means it's fully charged, any additional charge will just be converted to heat. Also, Li-ion batteries have protection circuitry inside the battery to prevent overcharge so the battery will just "refuse" any additional charge once the circuit says it's full.
The only advantage I can think of is that your batteries are charging while you are sleeping so you don't have to wait.