Charging overnight won't degrade really. But charging while using the phone surely will.
Every rechargeable battery has certain number of charge discharge cycles which is specific to its capacity (current output or voltage).
The Basis for charge discharge cycles is the Reversible reaction occurring in the battery.
- (Li+ + e- → ← Li-) ( → Charge ; ← Discharge)
Excessive charging will not help, in the other way it corrodes the electrode enhances the degradation rate of the electrolyte.
So, here's my suggestion - Use your phone from 100% to 5% and only then leave it for charging.
Don't take it to 40% and put it on charge and still keep using it. Battery temperatures can reach around 40°C while you're using your phone with the charger plugged in and that's not good for you battery life.
There are two main types: Lithium ion and Lithium polymer, both are having different chemistries and they are not same. Lithium polymer is light weight and can have more Coulombs* ratings, which means capable of giving more current than lithium ion. Lithium polymer is less likely to die quickly than lithium ion by over charging. But still both lithium ion and lithium polymer can be over charged by leaving charger on.
- 1C = 1A ⋅ 1s
- * The coulomb (unit symbol: C) is the International System of Units (SI) unit of electric charge. It is the charge (symbol: Q or q) transported by a constant current of one ampere (A) in one second (s):
Also turn brightness down, remove from your device unneeded apps and emails, check which apps consume most battery or are get alerts, and in areas of bad reception do a network reset. A lot of battery life especially on iPhones are due to bad upgrade and apps in the background using your resources.
- Don't use your phone while charging.
- Leaving it to charge all night is fine, given that you're following the rule above.
- Charge phone only when it is under 5% capacity
- Check what is running on your device.