Negative affects from leaving iPhone plugged in at 100%? I usually plug in my phone at night when I go to bed but it’s almost always at around 70-80% so am I damaging my phone (iPhone 6S) from having it plugged in at 100% for a few hours every night?
Questions about batteries and charging often risk inviting a lot of opinion, so to try and keep this to the facts, you should read what Apple has to say about their batteries. 

However, leaving your iPhone plugged into an AC source overnight causes no problems whatsoever. In fact, this is what millions of users do each night. 

The only time you need to be concerned about storing an iPhone at 100% is if you're going to be storing it long-term. On storing devices long-term Apple says:


  Do not fully charge or fully discharge your device’s battery — charge
  it to around 50%. If you store a device when its battery is fully
  discharged, the battery could fall into a deep discharge state, which
  renders it incapable of holding a charge. Conversely, if you store it
  fully charged for an extended period of time, the battery may lose
  some capacity, leading to shorter battery life.


Some other key points from Apple are:


Apple lithium-ion batteries use fast charging to reach 80% of their capacity and then switch to slower trickle charging to complete the charge.
Charge your iPhone whenever you want. Many people believe you need to let them discharge 100% before recharging. This is not true.
Ambient temperature is one of the biggest factors in battery health/life. iPhones are designed to work at their best when ambient temperatures are between 0° to 35° C (32° to 95° F).
Store your iPhone where the temperature is between -20º and 45º C (-4º to 113º F). Don’t leave it in your car, because temperatures in parked cars can exceed 45º C (113º F).