The brain is only about 2% of the body mass but uses around 20% or more of the body's energy. I suppose that this peak cognitive perfomance corresponds to the historical availability of calories. Until only a few generations ago, those slow periods and fast periods were times when food was quite scarce or abundant, the late spring hosting the "starvation moon" or "
hunger moon" and such. The late summer and early autumn were times of plenty. So on a guess, it might be that the brain's metabolism also slows down during the late winter and early spring when it was historically quite hard to get anything to eat.
