The Afternoon Slump: Sleep Pressure and Your Body Clock

Two systems that control sleepiness

Sleep researchers often explain sleepiness using the “two-process model” developed by Alexander Borbély. It describes a homeostatic process, called Process S, that builds up sleep pressure the longer you are awake and releases it during sleep. A second process, Process C, is controlled by the circadian clock and runs on a roughly 24-hour rhythm. The two interact continuously. [1]

The US National Heart, Lung, and Blood Institute describes the same idea in plain terms: a sleep drive that builds with time awake, and internal rhythms influenced by light and darkness. [2]

Sleep pressure: the hourglass

Think of sleep pressure like an hourglass that starts filling as soon as you wake up. The longer you stay awake, the stronger the need to sleep. A good night’s sleep empties it again; a short nap empties it a little.

This is why a nap in the afternoon can feel refreshing, and why a nap late in the evening can make it harder to fall asleep later: it releases pressure you would otherwise need at bedtime.

The body clock: alertness on a timetable

Your circadian clock sits in a small area of the brain and is synchronised mainly by light. It promotes wakefulness during the biological day and sleep during the biological night. The clock’s alerting signal gets stronger as the day goes on, which helps you stay awake into the evening even though sleep pressure keeps rising. [1]

Carrier and Monk describe how many types of performance, from reaction time to memory, change across the day as a result of both processes. [5]

Where the afternoon dip comes from

In the early afternoon, sleep pressure has already built up for several hours while the clock’s alerting signal is not yet at its strongest. Many people feel this as the post-lunch dip. Timothy Monk notes that it occurs even when people skip lunch and do not know the time, although a heavy, high-carbohydrate meal can intensify it. [3]

Experiments with an ultra-short sleep–wake schedule show the same pattern from a different angle. When people were allowed to try to sleep for a few minutes every 20 minutes around the clock, sleepiness had a smaller secondary peak in the mid-afternoon and a “forbidden zone” for sleep in the early evening. [4]

Why short nights add up

The hourglass explains why a single short night makes the next afternoon harder. But it gets more serious when short nights become a habit. In a well-known study by Van Dongen and colleagues, healthy young adults spent two weeks with 4, 6 or 8 hours in bed per night. Those with 6 hours or less built up steadily growing deficits in attention and memory. After two weeks, those deficits were comparable to one or even two nights without sleep. [6]

Remarkably, the participants’ own sleepiness ratings rose only slightly after the first days. In other words, people adapted to feeling tired, while their performance kept getting worse. [6] A nap can soften the afternoon slump, but it cannot pay back a long-term sleep debt.

What you can do about the slump

• Plan a short nap in the early afternoon if your day allows it. See the best time to nap.
• Get daylight in the morning and during breaks; light is the main signal for the body clock. [2]
• Watch your lunch. A lighter lunch may make the dip less pronounced. [3]
• Schedule demanding tasks outside your personal low point, if you can.
• Protect your night sleep. It is the foundation that a nap only supplements.

Morning types, evening types

Not everyone has the same timing. Morning types tend to feel the afternoon dip earlier, evening types later. Carrier and Monk point out that such individual differences affect when performance peaks and drops. [5]

So rather than following someone else’s schedule, observe your own pattern for a few days. When does your energy drop? When does it come back? That is the best basis for planning a break.

Common questions about the afternoon slump

Is the afternoon slump caused by food?

Not only. It is rooted in your sleep regulation, but a heavy meal can make it stronger. [3]

Does coffee replace a nap?

Coffee can make you feel more alert, but it does not reduce sleep pressure. Some people combine both in a coffee nap.

Can I reset my body clock with a nap?

No. A nap reduces sleep pressure a little; light and regular sleep times are what set your clock.