Jen, I am SO there with you! Although Sparky doesnt’ have to go away at night, I often go to visit Sweet Pea so I understand.
I have several theories:
#1 – When he is gone, you feel completely responsible for the family so you’re in overdrive to make sure they are taken care of. The burden rests clearly and squarely with you so you cannot sleep easily.
#2 – The comfort of having him around. I think spouses are soothing to our spirits (if you marry the right one) and you obviously did. Thus, when he is around, you are soothed to sleep by his presence. (Not to be confused with BORED to sleep!!!)