Usually I charge my iPhone when I go to sleep.
So most of the times I will overcharge...
Is this bad?
I disagree with what many have written in this thread. I believe that doing what you state above will negatively affect the lifespan of your battery.
However, the effect will be so small that it will be highly unlikely that you will notice it.
There is a tutorial on this someplace on the Internet. I'm sorry that I can't find it for you. But it made the most sense of what I've read on this subject.
Think of your phone's battery as a balloon that you are filling up with water and then emptying over time. Now if you fill up this balloon as much as you can and then leave it sit for several hours, you are placing a great deal of stress on the balloon material.
However, if you fill the balloon up and then once it is full you start to empty it over time, you immediately start to relieve the stress.
When you think about it, your battery doesn't store water, it stores electricity. The more elecitricity in the battery, the greater the strain on the materials in the battery.
The tutorial I read said that the best thing you can do for lithium ion batteries is to do small charges instead of long deep charges.
Think of the balloon again. It's better to say keep the balloon between 40% and 60% full than to drain it completely and fill it back up to 100%.
Now again even though I believe what I've written above, I don't think you would be able to tell the difference after two years. The battery is going to degrade either way. I think it unlikely you would be able to tell the difference if you followed my advice or if you continued doing things your own way.
By the way, I agree with what others have written that you can't overcharge your battery, but I still think you place additional stress on the battery by leaving it plugged in.
By the time you wear the battery out, you'll be ready for a new phone any way.