Anyone who's experienced falling spirits in autumn or a springtime bounce in the step knows that the changing seasons can affect mood. But can seasons affect the way your brain works overall? New research suggests that the answer is yes - and that time of year is just as important as time of day when it comes to how efficiently brains operate.

In a study published in the journal PNAS, scientists looked at the brain activity of participants deprived of seasonal cues over the course of a year and a half. Getting rid of seasonal cues is no small feat: During different time periods, participants slept eight hours in a completely dark room, then were deprived of sleep for 48 hours in a soundproof room with dim light and no way to tell the time of day. After a 12-hour sleep window, they were assigned two cognitive tasks, one for sustained attention and one for working memory. This went on for 4.5 days.

When researchers analyzed functional MRI readings of the participants' brains, they found a striking seasonal rhythm. Participants' abilities to perform different tasks remained relatively stable throughout the year. But the responses of their brains to the tasks varied by season. Brain responses related to tasks requiring sustained attention peaked around the summer solstice and bottomed out around the winter solstice. For tasks requiring working memory, participants' brain functions peaked around the autumn equinox and bottomed out around the spring equinox.

The research, which was performed in conjunction with another sleep study, suggests that although brains are capable of performing tasks with the same degree of accuracy throughout the year, the cost of cognition fluctuates from season to season. Researchers spotted seasonal variations throughout the brain and suggest that perhaps some cognitive processes are more linked to the environment than others.

Scientists have long known that other animals have built-in annual rhythms that dictate things, such as mating and hibernation, but those rhythms are much harder to track in humans. In humans, things such as Vitamin D levels and the immune system fluctuate by season. "These studies are really difficult to conduct," said Gilles Vandewalle, a Belgian sleep researcher who led the study. To prove true seasonality, scientists would have to hold participants in a lab for at least 13 months under extremely strict controls - a task that would be feasible, but tough.

Vandewalle was surprised by how strong the seasonal signals were with just 4.5 days without seasonal cues. If the study were repeated at different latitudes or in different countries, it could reveal whether factors, such as social interactions, humidity or temperature, affect brain function on a seasonal basis. The work also could reveal physical processes behind things such as seasonal affective disorder. "Now we can try to disentangle it," Vandewalle said - but he warned that just because brain function changes from season to season doesn't mean that, say, summer is a better time to pay attention or that fall is a better time to remember things. Seasonality is "not a problem for the brain," he said. "It's just the way it is."