It's not just you: It really is harder to think straight on hot days.

The hotter you are, the harder you'll find it to make complex decisions, according to a series of studies that suggest the problem might be caused by low glucose levels in the brain.

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Study details

Amar Cheema from the University of Virginia and Vanessa Patrick from the University of Houston conducted a series of studies to determine the effect of heat on a person's ability to make decisions.

For the first study, the researchers gathered sales data for different types of lottery games in St. Louis County. They found that sales of scratch tickets, which require buyers to choose between a number of options on each ticket, fell by nearly $600 for every one-degree Fahrenheit increase in temperature. Sales for lotto tickets, on the other hand, which require very few decisions for the buyer, didn't change at all.

In a separate study, the researchers asked participants to proofread an article in either a warm room, set at 77 degrees Fahrenheit, or a cool room, set at 67 degrees Fahrenheit. They found that the participants in the warm room failed to identify nearly half of the spelling and grammatical mistakes in the articles, while those in the cool room missed only about 25% of the mistakes.

In a third study, the researchers asked participants to choose between two cell phone plans in either a warm or cool room. One of the plans was structured to look attractive at first glance, but the other was actually more cost-effective. The researchers found that the participants in the cool room chose the better plan more than half the time, while the participants in the warm room chose the better plan only 25% of the time.

For their final study, the researchers placed participants in either a warm or cool room and asked them to choose between two products: one that was more traditional and one that was innovative. Participants in the warm rooms were much more likely to pick the traditional product over the innovative one.

Other research has produced similar results, including a report from the National Bureau of Economic Research that found test scores were lower during high-temperature days in schools without air conditioning.

Why it's so hard to think in the heat

The likely explanation for these results is glucose levels in the brain, Adrian Ward writes for Scientific American.

Glucose is involved in the brain's decision-making process, especially in mental functions that require significant effort. But glucose also is involved in regulating body temperature, providing the energy needed to either cool down or heat up. According to Ward, humans need more energy to cool down than to warm up, so in hot environments, the body uses more glucose—leaving less available for the brain.

Cheema and Patrick tested this hypothesis experimentally, depleting the glucose supplies of half of a study's participants before placing them in the warm or cool rooms. They found that the undepleted participants in the warm rooms acted in similar ways to the pre-depleted participants, suggesting that glucose depletion really does explain why people think worse in warm environments.

Taken as a whole, Ward writes, these studies suggest that temperature can have a noticeable effect on our patterns of decision making, as our bodies use up resources otherwise used for complex decision-making to maintain homeostasis (Skwarecki, Vitals, 6/5; Ward, Scientific American, 2/12/13).

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