iphone turning itself randomly off?

[mtj]

I have a 3g iphone, firmware 2.1 ... and lately when I leave my iphone overnight, I wake up to find it turned off ... it happens so far, when I dont plug it in to charge, but when I turn it back on, it still has 35+ battery... anybody else having this same issue?

[bholloway546]

Might be a bad battery. Does it turn of at all during the day when you're using it.

[mtj]

Nope^^ just at night when its inactive for some hours ... thats the only thing i can see that would prompt it, cause otherwise it works as it should.