Tell if battery in iphone goes to 20 percent from 40 percent in 8hrs?

Is this normal?

brightness is at zero, its on ios 6.1
location services are off, push notifications are off,
ar night 11.00 pm it was 40 percent, its morning 7.15 am and its showing me 20 percent, is this normal

It happens to me when I don't turn off the Wi-Fi/cellular. If you don't do that it sometimes drains the battery a lot quicker. If you've done all of those and turned off the cellular and Wi-Fi with airplane mode, your battery is dying. I don't mean dying as in losing time while using your iPhone, I mean your battery itself is physically losing the capability to hold a charge. I'd recommend taking it to a phone store or something near you if it still does this after the tip I gave.

Yes. Put it on flight mode when you are in sleep.