Long period of overcharging

Hello,

No definitely not.

Note: many people consider it a best-practice to put the phone on a charger overnight every night:

1. You start the new day with a fully charged phone
2. iCloud backups occur automatically (assuming a Wi-Fi connection)
3. Software updates occur automatically (assuming a Wi-Fi connection)
4. If something bad happens overnight, you know where your phone is and it is at least partially charged 

Also, see this for some more detail:

When to charge your iPhone or iPad

Good luck.

All apple devices cannot be over-charged. The device itself controls the charging of the battery, and it simply stops charging once the battery is at capacity. This is an industry standard for all SMART lithium batteries in all consumer devices (and has been for decades now).

As long as you are using an Apple-provided power adapter, you can leave your iPhone connected. These adapters are "smart" and know when to charge or stop charging.