Listen to the AI-generated audio version of this article. (Beta)
It’s half past nine in the evening, and the air in the flat is standing still. Outside the temperature has dropped, inside it hasn’t; the bedroom has had sun on it all day. Someone asks you a harmless question, and you hear yourself answer in a tone that doesn’t fit the question. Later you lie awake, turn the pillow to the cool side and are annoyed with yourself on top of everything else.
The next morning you’re tired, thin-skinned and slower than usual. And somewhere at the back of your mind sits the suspicion that you don’t have much of a grip on yourself at the moment.
That suspicion is wrong, and there’s a fairly clear chain of reasons why.
The Driver Who Leans on the Horn
That heat does something to mood has been studied for decades, and one of the best-known investigations took place not in a laboratory but at a junction in Phoenix, Arizona. Douglas Kenrick and Steven MacFarlane had a car sit still at a green light and measured how the drivers behind it reacted. The hotter it was, the sooner and the longer they sounded their horns.
Similar patterns turn up in many datasets, right through to analyses of crime statistics. Honesty requires adding that the cause is harder to pin down here than it first appears. On hot days more people are outdoors, out later in the evening, and there are simply more occasions for friction. How much of it the temperature itself explains and how much the changed circumstances is still argued over.
Something else, though, can be shown far more cleanly: what heat does to thinking.
Twelve Days in Boston
In the summer of 2016, a group around Jose Guillermo Cedeño Laurent at Harvard used an opportunity that rarely comes along. A heat wave was approaching Boston, and on campus students lived in two kinds of halls: some with air conditioning, some without. The same city, the same weather, the same daily life; only the indoor temperature differed.
For twelve days, every participant took two short tests on their phone each morning right after waking. Five days of ordinary weather, five days of heat wave, two days of cooling down. Among other things, the tests measured how quickly someone reacts and how well they can hold their focus on what matters when something else is competing for it.
The students without air conditioning did worse, across several measures, in reaction speed as much as in working memory. That on its own would hardly be surprising.
What was remarkable was when the gap between the groups grew widest. Not at the peak of the heat wave, but in the two days afterwards, when outdoor temperatures were already falling. The buildings without air conditioning were still hot inside. Their residents had long since had the feeling that the worst was over, while their heads were still in the middle of the strain.
That’s the first part of the explanation: you can’t tell from heat what it’s costing you, and you notice it least when you believe it’s behind you.
Why the Night Is the Real Problem
The stronger effect runs by a detour anyway, and that detour goes through sleep.
Falling asleep is a process that needs temperature. For the body to find its way into sleep at all, its core temperature has to drop by nearly a degree. It manages this by widening the blood vessels in the hands and feet and giving off the warmth through the skin. How well that outflow works predicts remarkably reliably when someone falls asleep. If the room is warm, it works badly, because the warmth has nowhere to go.
How large this effect is was shown by an analysis from Nick Obradovich and colleagues, drawing on reports from seven hundred and sixty-five thousand people. One degree of higher night-time temperature led to around three additional nights of too little sleep, per hundred people per month. The effect was strongest in summer.
A later study using sleep trackers, covering over seven million nights across sixty-eight countries, could also show exactly where it comes from. On warm nights, sleep gets shorter mainly because falling asleep is pushed back. You go to bed at the usual time, the alarm goes off at the usual time, and half an hour has gone missing in between.
What a Short Night Does to Your Patience
That missing half hour is where the tone you answered in comes from.
A group around Matthew Walker examined what a sleepless night does to emotional reactions. Participants in a scanner were shown images that grew steadily more unpleasant. In the rested group, the amygdala, the structure that flags anything threatening, responded as you’d expect. In the sleep-deprived group, that response was around sixty percent stronger.
More interesting still was the second finding. Normally the amygdala is closely connected with the prefrontal cortex, which sorts and dampens its reports. That very connection was weakened in the sleep-deprived group. So the alarm got louder, and the authority that usually puts it in perspective was harder to reach.
That closes the chain. The warm night pushes back your falling asleep. The missing sleep amplifies your reaction to anything unpleasant and weakens the brake at the same time. The next evening it’s warm again, and the harmless question meets a person whose alarm system is set more sensitively than usual.
Why It Feels Like It’s About Them
That explains why you’re irritable. What it doesn’t yet explain is why the irritation feels as though it had something to do with the person in front of you.
Craig Anderson and his colleagues put people in rooms of differing warmth. In the hot rooms, hostile mood rose, hostile thoughts came more readily to mind, and the body was measurably more aroused.
The most telling part of the result lies elsewhere. Those same people rated themselves as less aroused than the ones in the cool rooms. The body was running at higher revs while the feeling of being worked up was weaker. The arousal was there and stayed invisible to the person carrying it around.
This is where the psychology begins. Arousal without an explanation acquires one. The mind rarely reaches for the thermometer; it reaches for whatever is in the room, and that’s usually a person. An overheated room and a body running hard turn, in the experience of it, into a question that was out of order.
From his experiments Anderson concluded that heat mainly tips the appraisal of ambiguous social situations in a hostile direction. The difference from simply overreacting is a fine one, but it counts. The question arrives less neutrally than it was meant, before you’ve reacted to it at all.
What This Says About You
Nothing.
Someone who is thinner-skinned in August than in April hasn’t lost character. They’ve been sleeping badly for two weeks and are working during the day against a strain that runs in the background and isn’t experienced as a strain, because it looks like nothing more than summer.
That it nonetheless feels like character comes down to the same invisibility. From the inside, an overheated and undersleeping state doesn’t look like a state. It looks like a world in which people are being unusually difficult today.
This isn’t an excuse for everything. A curt remark stays a curt remark, and whoever made it can take it back. But it makes a difference whether you take the short fuse for a problem of personality or for what it mostly is in these weeks: the result of too little sleep in too much warmth.
An exercise for the coming week: the thermometer before the person
The first part applies to the moment itself. If something annoys you this week more than the occasion warrants, check the temperature before you take a closer look at whoever is in front of you. The point is to give the arousal a second possible sender. Saying to yourself once that it has been too warm here for days and that you’ve been sleeping badly because of it gives you an explanation that happens to be true, and you no longer have to look for one in the person opposite. The annoyance doesn’t disappear at that. It just becomes what it is again, and stops being a verdict on someone.
The second part applies to the evening. Everything that helps on hot nights comes down to making it easier for the body to give off warmth. Three things that fit into the hour before bed.
Cool the room, not yourself. If it’s cooler outside than in of an evening, airing the place through does more than anything else, and during the day the most useful thing is keeping the sun out from the outside, with a blind or a curtain, before it gets into the room. What’s once inside is hard to get out again.
Shower lukewarm rather than ice cold. That sounds wrong, but it follows straight from the mechanism. Cold water makes the vessels narrow, and the warmth stays in. Lukewarm water widens them, the body then gives off heat better, and the core temperature falls more easily. The brief freshness of a cold shower costs you afterwards exactly what you need in order to fall asleep.
And leave your hands and feet free. Most of the heat loss runs through them. A blanket covering only your feet prevents the very process that makes you sleepy.
If you snap at someone this week without there being a reason: how did you sleep the night before?
Sources
- Kenrick, D. T. & MacFarlane, S. W. (1986). Ambient Temperature and Horn Honking: A Field Study of the Heat/Aggression Relationship. Environment and Behavior, 18(2), 179–191.
- Anderson, C. A., Deuser, W. E. & DeNeve, K. M. (1995). Hot Temperatures, Hostile Affect, Hostile Cognition, and Arousal: Tests of a General Model of Affective Aggression. Personality and Social Psychology Bulletin, 21(5), 434–448.
- Anderson, C. A. (2001). Heat and Violence. Current Directions in Psychological Science, 10(1), 33–38.
- Cedeño Laurent, J. G., Williams, A., Oulhote, Y., Zanobetti, A., Allen, J. G. & Spengler, J. D. (2018). Reduced Cognitive Function During a Heat Wave Among Residents of Non-Air-Conditioned Buildings: An Observational Study of Young Adults in the Summer of 2016. PLOS Medicine, 15(7), e1002605.
- Obradovich, N., Migliorini, R., Mednick, S. C. & Fowler, J. H. (2017). Nighttime Temperature and Human Sleep Loss in a Changing Climate. Science Advances, 3(5), e1601555.
- Minor, K., Bjerre-Nielsen, A., Jónasdóttir, S. S., Lehmann, S. & Obradovich, N. (2022). Rising Temperatures Erode Human Sleep Globally. One Earth, 5(5), 534–549.
- Yoo, S.-S., Gujar, N., Hu, P., Jolesz, F. A. & Walker, M. P. (2007). The Human Emotional Brain Without Sleep — A Prefrontal Amygdala Disconnect. Current Biology, 17(20), R877–R878.
- Kräuchi, K. & Deboer, T. (2010). The Interrelationship Between Sleep Regulation and Thermoregulation. Frontiers in Bioscience, 15, 604–625.
