It’s been a hot July in a lot of the Northern Hemisphere! Research shows that when it’s too hot, the brain slows down. For instance, one recent study showed that students without air conditioning performed 13% worse on a set of cognitive tests than students with air conditioning. This finding is mirrored in other studies.
More details on VLRN.org: http://www.wlrn.org/post/heat-making-you-lethargic-research-shows-it-can-slow-your-brain-too