A curious pattern

 I have noted since about mid November that my pressures start jumping up right after sundown (6ish?) and then mysteriously drop around 8:45-9ish.  For example, they can be in the mid teens in late afternoon, and then after this time start up to an average of low 20's (with occasional higher spikes) during this time. Afterwards, they can drop to lower teens. Last night, for example, they went from about 16 pre sun down, got as high as 22 and then were 11 by 10pm! During the high period, even exercise won't bring them down.