What makes my gums start bleeding literally overnight?

Gingivitis: You may have undiagnosed gingivitis that should be checked and confirmed by your dentist. Another possibility is that you are a mouth-breather. Your gums in the front of your mouth become dessicated during the night and that can also lead to some bleeding.

Gingivitis, Periodon: It may be a problem that has existed, but went unnoticed. See a dentist for an evaluation and possible treatment.