Iphone 3GS Battery issues frequently?

Hi, I'm using iPhone 3GS from past 6 months. Frequently i'm facing issues from battery. Such as normal when the battery on my iPhone is (for example) 77-80%… Etc when i sleep at night & wake up at morning i see that battery decreases to 55-50% and that too in stand-by mode. And I will keep on closing all the app which are running in background and wi-fi also turn off & brightness is too less due to battery life I've reduced to very low even throught i'm having issues for battery. So please let me know what is the mail problem by this sort of issue as i frequently suffer from.

I also wake up with my iPhone battery low (usually about 10% lower). I think this is because your phone might be automatically updating emails and other apps throughout the night, causing the battery to decrease. To prevent this, make sure all your apps are closed out of and possibly consider shutting down your iPhone before bed every night.

All phone batteries die pretty fast.