Hi
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This could have been due to a nose bleed or due to bleeding from the mouth, from gut or due to coughing out blood during sleep. It is difficult to determine the cause on net. Consult an ENT specialist and your PCP regarding this.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!
You may have biten your lip or tongue while sleep which many people do often and usually result in wearing some type of mouth gard.
I agree with what coccygectomblues said.
it could be due to a nose bleed, biting your cheek during sleep, or something more serious like an ulcer. you should make an appointment to see your doctor if you can rule out a cut in your mouth.