Sleep, stress, allergies, level of distraction, foods, medicines, supplements. sound levels and frequencies, jaw, cervical condition, ... Everybody is different so you need to find your sweet spot.
@Joe Cuber it probably is the same all day, maybe just some days I am busier than others. Any time I'm sitting around, trying to nap, watch tv, read a book, or relax it is the most noticeable. =(