In our house the routine was always for me to stay up with the babies and my husband got up with them early in the morning. So I got on that cycle....I go through bouts of insomnia as well and have since I was in high school.
I can function pretty well on only 5 hours sleep but less than that and I crash around mid day.
What is funny is with as quiet as the house is during the day - I still get more done at night when everyone is asleep - guess its my bodies cycle used to doing work at night or something!