2.3 gingerbread - How can I prevent the battery in my Nexus S from draining overnight?

I have a Google Nexus S and for the past 2-3 days whenever I go to sleep the battery drains completely overnight. I mean when I go to bed the charge on my phone is plus 90% but for the past 2 days it's dead when I wake up. Thankfully I didn't miss work these two days otherwise my boss is gonna kill me.

I have Android 2.3.7

Any advice?