If I stayed in a chin toward chest position for very long, my autonomic nervous system would start to malfunction. Low blood pressure, slowed pulse, maybe even apnea. I would sometimes stop breathing while awake. It took more than a year of neck mobilization and good sleeping posture for my neck to strengthen and stabilize before I could do anything with my head tipped forward for long. I believe it was inflammation that interfered with blood flow and nerve function in my brain stem area.