Why is it

Why is it that when I am home and need to make dinner or tend to Olivia or just sit down for a minute Zachary takes a 30 minute late afternoon/early evening nap but when I go out for the evening he sleeps for several hours? I am not complaining that last night when I finally had a reason to leave the house by myself that he slept well I just wanted Rob to get a little time in with both kids since he has been so busy. I left just before 7 after putting Zachary down around 6:30 and he woke up minutes after I returned at 9, which of course meant we didnt get him down for the night until close to 11. Feeding baby food to a baby at 10:30 is not my idea of fun! He did sleep until just after 7 this morning though so I did get some uninterupted sleep which was nice.