Mood swings, but no roundabouts

As the trip progresses I am noticing a daily swing in my mood.  Cycle touring is a rollercoaster of emotional peaks and troughs, but strangely on this trip I seem to be having a daily cycle!

I am always in a good mood at breakfast, I normally have a coffee and something to eat before packing up and hitting the road.  My mood remains good* until about 1-2pm when I try and stop for lunch (where facilities allow).  

For reasons I can't explain, my mood seems to drop in the afternoon and I cycle along feeling like a bit of a fragile emotional wreck.  However, this mood seems to pass after about an hour!  My mood in the evening tends to be great, especially once the tent is put up, a shower is had and food is on the go.

I have cycled enough to know that you have low points and if you keep going they pass, I have just never noticed it as a daily cycle (excuse pun).

*Subject to weather / road conditions!