4 Features of Summer That Cloud Your Thinking

Sunlight, heat, dehydration, and smoke can quietly impair cognition.

Posted March 27, 2026 | Reviewed by Tyler Woods

As summer approaches, you may find yourself forgetting things, struggling to concentrate, or generally feeling cognitively slower. You may wonder, “Is this normal? Does this happen to everyone?”

In past summers, this has certainly happened to me. For instance, I’ve struggled with more tip-of-the-tongue words than usual. The ever-elusive names—but also common words. Words like…what do you call that thing you put on the dog? Not a collar—the one with four straps? (Two minutes later): Harness.

Is there something about the summer months that causes worse brain fog and a sluggish memory?

Research shows the answer is yes. In fact, scientists have identified four key factors. And, fortunately, if you’re aware of these harmful conditions, you can mitigate their effects.

Feature #1. Longer Days—And Shorter Nights

Our brains require a steady diet of quality sleep to perform optimally.

But some research has shown that sleep suffers in the summertime. A study done in the Netherlands revealed that sleep duration—measured objectively with wrist-worn activity monitors—was shorter in summer than winter.

In another study , researchers in Germany examined brain activity during sleep across 12 months of the year. Although they found that overall time asleep did not change across seasons, REM sleep did. In this study, REM sleep was reduced in spring and early summer and increased during the winter months.

And the REM periods of sleep are known to be important for some memory and thinking skills, such as consolidating emotional memories in long-term memory .

What’s causing poor sleep in the summer? A major determinant seems to be extended daylight.

For example, a study conducted in the Arctic summer—with some periods of 24-hour sunlight—showed that extremely long daylight was associated with disrupted circadian rhythms , delays in melatonin secretion onset, and worse sleep.

Feature #2. Hotter Temperatures

Here’s a sentence I never envisioned writing: Maybe Mel Tormé was right: It’s just tooooo darn hot.

Do sweltering temperatures also diminish our thinking abilities?

A group of scientists in the UK investigated this. They brought volunteers into the lab to measure their mental agility in two thermal conditions: room temperature (70 °F) and an oppressively hot environment (103 °F).

In the heat, participants showed worse performance on a computer task that required inhibitory control—the ability to ignore distractions and stay focused. That’s an ability we often take for granted—yet we rely on it constantly in daily life.

More broadly, studies show that high temperatures impair both simple and complex cognitive tasks. However, it’s much worse when doing something more demanding—activities like doing mental arithmetic, planning how to solve a problem, and driving.

Feature #3. Dehydration

Another factor in summer’s assault on thinking is dehydration. Summer brings two key risks: the aforementioned hotter temperatures and greater physical activity .

Abundant evidence shows that dehydration impairs cognition . And importantly, the drop in performance is more extreme in high-level cognitive tasks, such as those measuring attention or logical reasoning, compared to low-level tasks, such as a simple reaction time measure.

The good news is that rehydrating with water reverses dehydration-associated dips in cognitive function , including performance on memory and cognitive flexibility tasks.

Feature #4. Wildfire Smoke

I live in the Pacific Northwest. Here, as in many places, our summers are getting increasingly smoky from wildfires.

I personally seem to have more headaches on really smoky days, and I wondered if these pollutants were affecting my brain and thinking.

Unfortunately, they probably are. A recent review showed that our brains are impacted by extreme heat, wildfire smoke, and the combination of the two. Specifically, these conditions are associated with inflammation in the brain and disruptions in neuronal signaling (the basis of all thinking).

For example, Cleland and colleagues documented a link between smoke levels and reduced focused attention. They followed a large group of people for a year, tracking their scores on a test of selective attention and smoke levels. The two measures matched up. As concentrations of wildfire smoke and particulates went up, focused attention scores went down.

Maybe we’d better step up those Smokey Bear ads again.

Of course, your specific countermeasures to summer’s harmful effects on thinking will depend on where you live and your lifestyle variables (to some degree—no pun intended). But here is a buffet of advice to choose from.

An article similar to this was published on the Psychological Science Lite blog: Blog - S. L. Penningroth

Centers for Disease Control and Prevention. (2024, April 15). Safety guidelines: Wildfires and wildfire smoke . U.S. Department of Health and Human Services. https://www.cdc.gov/wildfires/safety/how-to-safely-stay-safe-during-a-wildfire.html

Cleland, S. E., Wyatt, L. H., Wei, L., Paul, N., Serre, M. L., West, J. J., Henderson, S. B., & Rappold, A. G. (2022). Short-term exposure to wildfire smoke and PM2.5 and cognitive performance in a brain-training game: A longitudinal study of U.S. adults . Environmental Health Perspectives, 130 (6), Article 067006. https://pmc.ncbi.nlm.nih.gov/articles/PMC9196888/pdf/ehp10498.pdf

Garriga, A., Sempere-Rubio, N., Molina-Prados, M. J., & Faubel, R. (2022). Impact of seasonality on physical activity: A systematic review . International Journal of Environmental Research and Public Health, 19 (1), Article 2. https://doi.org/10.3390/ijerph19010002

Malcolm, R. A., Cooper, S., Folland, J. P., Tyler, C. J., & Sunderland, C. (2018). Passive heat exposure alters perception and executive function . Frontiers in Physiology, 9 , Article 585. https://doi.org/10.3389/fphys.2018.00585

Paul, M. A., Love, R. J., Hawton, A., Brett, K., McCreary, D. R., & Arendt, J. (2015). Sleep deficits in the High Arctic summer in relation to light exposure and behaviour: use of melatonin as a countermeasure. Sleep Medicine , 16 (3), 406-413. https://www.sciencedirect.com/science/article/abs/pii/S1389945715000398

Seidler, A., Weihrich, K. S., Bes, F., de Zeeuw, J., & Kunz, D. (2023). Seasonality of human sleep: Polysomnographic data of a neuropsychiatric sleep clinic. Frontiers in Neuroscience, 17 , Article 1105233. https://doi.org/10.3389/fnins.2023.1105233

Stachenfeld, N. S., Leone, C. A., Mitchell, E. S., Freese, E., & Harkness, L. (2018). Water intake reverses dehydration-associated impaired executive function in healthy young women . Physiology & Behavior, 185 , 103–111. https://doi.org/10.1016/j.physbeh.2017.12.028

Taylor, L., Watkins, S. L., Marshall, H., Dascombe, B. J., & Foster, J. (2016). The impact of different environmental conditions on cognitive function: A focused review . Frontiers in Physiology, 6 , Article 372. https://doi.org/10.3389/fphys.2015.00372

This article is part of the Bringwise Psychology Journal — daily insights on human behavior, mental health, and personal growth.