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

Its on ios 6.1
screen brightness i set at zero when i go to sleep
location services, push notifications are off,
i stopped running background apps at night,

at 11.00 pm it was 40 percent, now its morning & its showing me 20 percent,
is this normal?

It is the biggest complaint about all iOS devices, please go for Lumia series 620, 720 nice camera, good battery life, no lagging, good Data speed only problem lack of apps windows store