I have the habit of leaving my phone in charge all night while I'm sleeping, but I've read that this is going to make the battery last less time. Would it be possible to automatically stop charging the battery when it reaches 100%? I have a Samsung Galaxy A3 with Android Kitkat.
All phones automatically stop charging once they reach 100%. They then carefully monitor and "bounce" the battery level to ensure it doesn't drain too much, but doesn't overcharge, either. But they're definitely designed with this core concept in mind to reduce damage. Besides, if it stopped charging it at 100% and didn't start back up again, odds are by the time you woke up you'd need to start charging again to get back the 20-30% you lost, which is another charge cycle, causing exactly what you were trying to avoid.
You could buy any wireless plug and switch like we have some of the products from belkin. You can set timer when you want to stop them through your phone itself.