Why do my allergy symptoms improve when I go to sleep?

It really depends on what kind of allergen is causing the symptoms, but it sounds like it's a type of pollen. Of course, when he's out and about during the day, he's going to be meeting that pollen, but when he's in bed, he's likely not to. The exception for this is whether he or his partner has long hair. I often find people with pollen allergies have terrible problems at night because one of them has very long hair. What we know about long hair is that it is a complete static pollen magnet. So if you wash your hair before you go to bed rather than when you get up in the morning, you can spend an allergy-free night without having a cloud of pollen around you.