Nope, charging your iPhone overnight will not affect its battery life.
Myth: Leaving your iPhone overnight on charging will harm your battery life as the charger will keep pushing electricity into an already full battery. It will cause the phone to overheat and reduce life of its internal circuitry too.
Truth: Overnight charging does not harm your iPhone in any way. iPhone’s hi-tech circuitry in Lithium-Ion battery (for that matter 90% of other phones too have Li-Ion batteries) has a charging circuit that cuts off charging power when the battery reaches 100%.
So your phone actually switches the power source from battery to direct power from charger once the charging reaches 100%. Hence your battery rests while it is charging beyond 100%.
These myths were a truth for the older Nickel-Cadmium (NiCad) batteries which actually used to get overcharged and in turn became super hot. This does not apply to Lithium-Ion batteries, however the myth still hounds most of us.