pete - yes, I know exactly what you mean, but no, the spikes can happen at any time of the day or even night...I have a very good bead on my traffic since I use audio doorbells on key pages. I can hear my traffic while I work on other stuff. When it gets quiet, I know something is haywire.
I've been doing this audio monitoring for several years, and it indicated everything was running like clockwork 24/7, right up until May 18th...then a decent recovery, followed by the September 1st upset. I hope the yo yo comes back up!