A sleep study involving 28 participants had them follow a controlled sleep/wake schedule for three weeks before staying in a sleep laboratory for 4.5 days, during which time they experienced a cycle of sleep deprivation and recovery in the absence of seasonal cues such as natural light, time information and social interaction. The same participants went through this entire procedure several times over some 18 months. Brain activity was assessed while participants undertook an n-back working memory task, and a task that tested sustained attention.
While performance on these tasks didn't change with the seasons, the amount of effort needed to accomplish them did. Brain activity involved in sustained attention (especially in the thalamus, amygdala and hippocampus) was highest in the summer and lowest in the winter. Brain activity associated with working memory (especially the pulvinar, insula, prefrontal and frontopolar regions), was higher in the fall and lower in the spring.
Seasonality, therefore, could be one factor in cognitive differences that occur for an individual tested at different times.
Participants were healthy young adults; it would be interesting to see if the same results are found in older adults. It's possible that the effects are greater.