bleeding in third trimester only at night

I've had two bleeds with bright red blood. The first at 34 weeks around 3:30am and I went to the hospital and then did and us and monitored me overnight. They didn't find a cause and sent me home to rest. I had another bleed at 36 weeks at 5:00am followed by a hospital visit. There was no cause mentioned and myself and baby were deemed fine. I was just wondering if anyone had an idea as to why they only occur when I'm sleepin? I forgot to ask my doctor and have to wait a week until my next appt.