There are quite a few studies that suggest the brain performs better when it's cooler.

Three are discussed here. ("Cool room" was 19 C, "warm room" was 25 C.) http://www.scientificamerican.com/article/warm-weather-makes...

TL;DR: One study suggests ability to recognize spelling and grammar mistakes improves in a cool room. A second study suggests ability to do math and make a correct decision based on it improves in the cool room. The third study suggests people in a warm room prefer to avoid complex cognitive decisions and rely on simpler patters of decision making even when it resulted in inferior choices.

YMMV. Empirically, I focus and perform better when a room is 18 - 19 C (below "room temperature", which seems to be 20 to 22 C). I am Canadian.