Low Oxygen Saturations in Hypoplastic Left Heart babies while sleeping

Hello,
  My 8 month old son has hypoplastic left heart syndrome and the acceptable oxygen saturation range for this condition is 75 to 85% O2.  During the day he stays within this range, but during the night when he sleeps he drops into the 60s.  We put supplimental oxygen on him and with 1.5 L of supplimental oxygen, he may come into the high 60s to low 70s (no more than 73).  Is this ok for his condition?  He has had his Glenn surgery, but still has a BT shunt and so they also put in a PA band between his Glenn shunt and his BT shunt therefore the Glenn feeds the right lung and the bt the left.  Any input you can give me would be greatly appreciated.  Also, what is a normal drop in O2 while sleeping in a healthy person?  
Thank you,
Tammy