Your phone is therefore getting 'trickle charged' through the night — dipping very slightly then getting topped up again. That's one of the reasons why it's important that you use the charger that came with your phone whenever possible, otherwise there's the risk of causing unnecessary damage to it.
MicroUSB has standardised the system somewhat, but it's still not worth the risk of something catching fire. If you're using the supplied charger and cable then much of the power management will be taken care of for you.
Besides the issue of energy drain, keeping a phone topped up at 100% for longer than necessary degrades the battery slightly faster. The difference isn't huge, but there is a difference: lithium ion batteries prefer not to be fully charged if possible.
Shallow discharges, where the battery drains for a short time and then is charged for a short time, work better than leaving it on charge constantly — it's a bit like someone going for an occasional walk rather than sitting slumped on the sofa all day long.
The smarter battery future
As we've already pointed out, a lot of great work has already gone into making chargers as reliable and as energy efficient as possible, whether or not a smartphone is attached to them, that's another reason not to cut corners when it comes to getting replacement chargers.
Technology developed by Californian startup Qnovo is promising to reduce the time it takes for devices to charge and increase the lifespan of batteries at the same time. We're also seeing batteries that can be recharged from your body's movement, doing away with power sockets altogether.
In the not-too-distant future, charging times could eventually get down to just a few minutes, which would make overnight charging a thing of the past. So while unplugging your phone after charging (and unplugging the charger too) can make a small difference to the health of your battery and your electricity use, it's a problem that's already close to being solved.
That's good news for the planet, the device that sits in your pocket and your household energy bills. In the meantime: consider changing up your nightly charging schedule.