The afternoon slump: what causes it and what research shows

Do things get a bit of a slog after lunch? Research shows that the ‘afternoon slump’ really does exist: for some people and when carrying out certain tasks, there is a dip in energy in the middle of the afternoon, which is linked to a greater tendency to feel sleepy at this time of day. Here you can read about what’s behind it, the role lunch plays, and what studies reveal about short naps, bright light, exercise and the timing of caffeine intake.

What lies behind the afternoon slump

In research, this phenomenon is known as the ‘post-lunch dip’. According to the US researcher Timothy Monk, it is a real phenomenon and can even occur when someone hasn’t eaten lunch at all and doesn’t know what time it is. The roots of this lie in human biology; the slump may be linked to a 12-hour rhythm of the body’s internal clock. It may affect self-confessed morning people more than others.

Sleep researchers at Loughborough University describe this slump as a ‘bi-circadian’ phenomenon – in other words, part of the circadian rhythm that has little to do with lunch – and which is more pronounced after a restless night. So anyone who has slept poorly is more likely to feel the effects of the afternoon more keenly.

What role does lunch play?

What you eat does matter, but the evidence from studies is mixed. In a driving simulator study, twelve young men drove along a monotonous route for two hours after a night’s sleep shortened to five hours. After a heavy lunch containing 922 kcal, they veered out of their lane more frequently due to drowsiness than after a light lunch containing 305 kcal. The difference only became apparent after the first 30 minutes of the journey.

When it comes to the composition of the meal, the picture is less clear: in a study involving 16 participants, it made no difference whether the meal was high in fat or high in carbohydrates – around 1.5 hours after eating, everyone felt sleepier than before. Monk, on the other hand, writes that a high-carbohydrate lunch intensifies the slump.

Naps: How long is a good length?

In an Australian study, 24 young adults who slept well and did not otherwise take an afternoon nap had slept for only about five hours the previous night. At 3 pm, they slept for varying lengths of time.

• Compared to not having a nap at all, a 5-minute nap made hardly any difference.
• Within 10 minutes, all measured parameters – including drowsiness, tiredness and test performance – improved immediately, in some cases for up to 155 minutes.
• 20 minutes: The improvements only began 35 minutes after waking up and lasted for up to 125 minutes.
• 30 minutes: Immediately afterwards, alertness and performance were initially lower, but then improved – for up to 155 minutes.

The researchers consider 10 minutes to be the most effective duration overall among those tested. The findings apply to people who had not slept enough. According to the IQWiG, there is as yet no clear research evidence on whether naps are generally beneficial or harmful – for example, in terms of night-time sleep.

Bright light in the afternoon

In a Japanese laboratory study, 16 women aged between 33 and 43 sat by a window for 30 minutes after lunch, in light levels averaging around 3,260 lux. According to two measurement methods, they were subsequently less drowsy for at least 60 minutes than when exposed to less than 100 lux; there was no difference in the reaction time test. The researchers see this as a potential solution for workplaces where taking a nap is not possible.

A second study reaches a similar conclusion: After two nights of five hours’ sleep each, 16 adults aged between 18 and 35 felt less sleepy when exposed to bright light of 1,000 lux from midday until 5 pm, and performed better in an attention test than the control group, which was exposed to less than 5 lux.

Move briefly

In an earlier study involving 18 volunteers, the participants rated their energy levels as higher and their tension as lower for two hours following a ten-minute brisk walk.

A recent study asked 30 adults who spend a lot of time sitting to sit for six hours – once without a break, and once with six hourly walking breaks of 5 minutes each on a treadmill. With the walking breaks, they rated their energy levels higher, their mood was better and they felt less tired at the end of the day. Their performance on cognitive tasks did not change. Several co-authors worked for Johnson & Johnson.

Breaks are part of the working day

The Working Hours Act stipulates rest breaks for employees: at least 30 minutes for shifts lasting more than six but no more than nine hours, and 45 minutes for shifts lasting more than nine hours. These breaks may be split into periods of at least 15 minutes, and no one may be required to work for more than six consecutive hours without a rest break.

Part of the break can be spent enjoying bright light by the window or outside, and taking a few steps – precisely the two things that were examined in the studies mentioned above.

Does sugar help combat the slump?

It’s tempting to reach for something sweet, but the research doesn’t support this. A meta-analysis of 31 studies involving 1,259 participants found no positive effect of carbohydrates on mood at any point after eating. In fact, in the first hour after consuming carbohydrates, participants were even more tired and less alert than after taking a placebo.

In the study involving 18 volunteers, the participants initially felt more energetic after eating a chocolate bar, but an hour later they felt more tired and tense.

Caffeine: pay attention to the timing

Caffeine is absorbed quickly. According to the BfR, the stimulating effects can set in 15 to 30 minutes after consumption and last for several hours; the duration varies from person to person. The half-life in adults averages around 5 hours.

That is why timing is important. According to the EFSA, even single doses of 100 mg can affect sleep duration and sleep patterns in some adults, particularly shortly before going to bed. In a sleep study, 400 mg of caffeine shortened sleep by more than an hour even when taken 6 hours before going to bed. The IQWiG advises that, if you have trouble sleeping, you should avoid coffee, tea and other stimulants for 4 to 6 hours before going to bed.

By way of comparison: a can of Energy – Blackcurrant & Pomegranate contains 105.6 mg of caffeine, i.e. 32 mg per 100 ml, and bears the warning ‘High caffeine content. Not recommended for children or pregnant or breastfeeding women’. How long caffeine remains in the body is explained in ‘Half-life of caffeine’; the guideline values can be found in ‘How much caffeine a day?’. You can go without caffeine by trying these coffee alternatives.

And what about drinking?

The studies mentioned here have not investigated whether drinking helps to combat the afternoon slump. The fluid intake calculator shows how much you should generally drink: the DGE recommends 1,410 ml of fluids per day for adults aged 25 to under 51.

Key points at a glance

1. Keeping an eye on sleep: after a restless night, the low point is more pronounced.
2. Don’t eat too heavily at lunchtime: in a driving simulator study, a heavy lunch was found to increase drowsiness.
3. A short nap, if possible: in one study, 10 minutes proved most effective; after 30 minutes, alertness initially declined.
4. Bright light: by the window or outdoors – studies have shown that drowsiness in the afternoon decreased.
5. Brief exercise: 10 minutes of brisk walking or 5 minutes of walking every hour were found in studies to boost self-reported energy levels.
6. Caffeine early enough: the closer it is to bedtime, the more your sleep suffers.

You can find out more about energy no sugar on the ‘Energy no sugar’ page; the ‘Morning routine’ section shows you how to get your day off to a good start.

Frequently Asked Questions

Sources

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