Why do I crash at 3pm? The science behind the 'afternoon slump' - Ireland News Beep

Analysis: It’s tempting to blame the afternoon slump on a big lunch, but there’s actually some science and biology behind the 3pm crash

By Michelle Spear, University of Bristol

You’ve just finished eating lunch and you’re back at your desk, ready to dive into the rest of the day’s work. Yet by 3pm, you find your eyes starting to droop. Before long, the only thing you can think about is how much you’d like to take a nap.

The “afternoon slump” (also sometimes called the “3pm slump”) is a common experience. It’s that distinct drop in energy that tends to occur in the mid-afternoon hours. While it’s tempting to blame this sudden urge to snooze on what you’ve eaten for lunch, this alone can’t take the blame. The afternoon slump is actually caused by several distinct biological pressures beginning to converge.

One part of the explanation begins long before lunch. From the moment we wake up, our drive to sleep gradually increases. Adenosine – an organic compound involved in regulating sleep and wakefulness – accumulates in the brain while we’re awake, contributing to our gradual need to sleep, referred to as “sleep pressure”.

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Yet the reason we do not simply become progressively sleepier from breakfast onwards is because another system is working alongside it. Our circadian clock, which controls our body’s sleep-wake system, generates a changing drive for wakefulness across the day.

During the morning, this system helps to counteract increasing sleep pressure. But in the early afternoon, we enter a natural period of increased sleep propensity. As sleep pressure has continued to build regardless, this creates a window in which tiredness becomes easier to feel.

Lunch can then add another layer. Eating changes signalling between the gut, metabolism and brain, as the brain receives information about stomach distension, nutrients and gut hormones. The size and composition of the meal can also influence how we perform afterwards. Larger lunches have been associated with poorer attention and greater post-meal sleepiness – although it’s not clear that any particular food group, such as fat, carbohydrate or sugar, is specifically responsible.

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So a very large lunch may exaggerate the slump, but there’s no convincing evidence that one particular macronutrient is universally to blame.

We’re still teasing apart exactly how eating contributes to the mid-afternoon slump, but one intriguing candidate is the brain’s orexin system. Orexin-producing nerve cells in the hypothalamus (a region which regulates basic body function) help sustain wakefulness and are sensitive to metabolic signals, including glucose. Eating may therefore influence some of the neural circuits responsible for keeping us awake.

Recent research suggests that this post-meal sleepiness may also be neurologically distinct from simply being short of sleep. In a study measuring brain activity and vigilance before and after eating, post-meal drowsiness was associated with a shift in activity within the brain’s cerebral cortex – the outer layer of the brain that’s involved in attention, perception and complex cognitive processes. This shift pushed brain activity towards inhibition and slower reaction times.

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Sleep deprivation, by contrast, was associated with greater cortical excitability – meaning that neurons in the cerebral cortex were more readily activated. We can therefore experience something that feels remarkably similar to sleep deprivation – tiredness, slower thinking and a desire to stop concentrating – while the brain may have arrived there by different routes.

What you shouldn’t and should do

While many people make themselves a cup of coffee or tea, this isn’t a great strategy for overcoming the slump. Caffeine does not give the brain more energy. It works largely by blocking adenosine receptors. Although caffeine may temporarily block these sleep-promoting signals, adenosine will still continue accumulating. This is why caffeine can improve alertness but cannot substitute for sleep.

Timing matters too. A late-afternoon caffeine hit may solve one problem, but if enough caffeine remains circulating in your system by bedtime, you may struggle to sleep. This may cause even more fatigue the next day.

A better way to push back against the afternoon dip is using natural light. Light is a powerful signal to the systems that control alertness. A 2024 systematic review of 62 experimental studies found daylight most consistently improved subjective alertness and working memory – although effects on sustained attention were less consistent.

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The effect is not limited to natural light, either. One study found that using a lamp which simulated the blue light wavelengths of natural daylight improved reaction times, lapses in attention, sleepiness and motivation in office workers.

For those spending afternoons indoors, getting a bit of daylight (or simulated daylight) may help to wake you up. But if coffee and bright light isn’t enough, there is another option: stop fighting the sleep pressure.

A short afternoon nap can take the edge off the slump. Around ten to 20 minutes is often enough to reduce sleepiness and improve alertness while keeping you largely in lighter sleep, reducing the chance of waking with the grogginess known as sleep inertia. Set an alarm and aim to nap in the early afternoon rather than late in the day, when sleep may interfere with bedtime. Trial and error can help find your own optimal duration.

Attention, alertness and reaction time fluctuate according to biology as well as workloa

Longer naps of around 60 minutes may offer additional benefits – including for memory – but they increase the chance of waking from a deeper sleep feeling temporarily worse.

Perhaps the most useful response to the afternoon slump is not to treat it as evidence of laziness or declining motivation. Human cognitive performance is not designed to remain perfectly flat across the working day. Attention, alertness and reaction time fluctuate according to biology as well as workload. We have simply constructed working days that often assume otherwise. The Conversation

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Michelle Spear is a professor of Anatomy at the University of Bristol. This article was originally published on The Conversation.

The views expressed here are those of the author and do not represent or reflect the views of RTÉ

Why do I crash at 3pm? The science behind the ‘afternoon slump’

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