Your immune system has rhythms. There are a whole zoo of different immune cell types in your blood. The immune system is like a bunch of liquid organs, each with many cell types. Many cell types which are derived from bone marrow have a tendency to return to the bone during the day. They go home. Circulating cells found in a blood draw, such as neutrophils, can increase by 2 fold at night. At night more immune cells are on active duty.