I don't know if it's normal, but I experience pretty much the same cycle (and have for the past 5-6 years) -- for me, what's helped is:

- being able to anticipate it

- using that time period to do more introspective stuff like journalling (instead of trying to be as productive as I usually am)

- trying to get outside/working out

- and sometimes drinking tea for the l-theanine + caffeine boost (which tends to snap me out of it for a duration of time)