Can I safely charge my iPhone 4 overnight on a regular basis?
All this hype about how to charge your batteries is blown out of proportion.

Although some batteries (I forget which types exactly) can have longer lives if charged at "optimal" rates and temperatures, the reality seems to be that it does not extend the life by much. One of the research papers I read concluded something like if you charge your device at around 17 degrees from 0% until 75% full you can extend the lifespan by about 1/5 of the total life span.

I have charged my devices for days at a time, and my iPod Nano has been plugged in for months, and holds a charge the same as my other nano (I have two) which almost never gets used and only charged ocasionally.

In conclusion, just do whatever is convenient for you.

EDIT: Here are some links to the apple pages related to their iPhone batteries:


Batteries - iPhone
iPhone Battery Replacement Service: Frequently Asked Questions