Charging Your iPhone Overnight Is Bad for Its Battery
FALSE: This is one of the most common misconceptions. iPhones are designed specifically not to overcharge. Inside of any MFi-certified (Made For iPhone/iPad/iPod) cable is a chip. This chip is designed to control the input of power to the devices.
If you leave the device plugged into a MFi-certified charger overnight, there will be no detectable damage done to the battery and it will not cause the battery to degrade faster.
However, if you do plan to be away from the device for a long period of time, you should charge the device to 50%, turn it off and leave it in a dry area that will not exceed a comfortable temperature. If you plan to store it for a very extended period of time, recharge the device to 50% every six months.