[nonameiguess]

Just to add to the chorus of modern reasons this might happen, when I was going through pretty bad spinal disc degeneration, I almost always woke up after somewhere between 3-4 hours of sleep. I just couldn't go any longer until I was in too much pain and needed to move about, ice it, take more meds, whatever I could do. That isn't the case any more because of spinal fusion surgery that wasn't an option in the medieval era. I'm sure pretty bad pain you just kind of tried to ignore but could never really get rid of was a common thing back in the day when corrective surgery didn't exist and reliable pain meds didn't either. Alcohol helps, but also disrupts sleep.