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The clock within

At three in the morning and four in the afternoon, alertness drops, even if you have slept. It is not chance and it is not weakness. It is a clock the body carries within it, and willpower cannot move it. Pick a time and watch what happens.

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Published: 24.09.2026

Scenario

Your day

Wide margin Past the threshold Asleep Peaks in sleep-related crashes

Up at 07:00, in bed at 23:00. The colour of the ring shows how much margin your body leaves you, hour by hour.

01Light

Morning sets the clock right

The internal clock does not run on an exactly 24-hour cycle. Measured in a laboratory, in very dim light, one cycle lasts slightly longer: 24 hours and 11 minutes on average. Left uncorrected, it would fall a little behind every day. Czeisler et al. 1999

What sets it right is light. In the retina, besides the cells we see with, there are others that contain a substance called melanopsin. First found in rats and mice, they also exist in the human retina. They are not for seeing: they measure how much light there is, especially blue light, and send the signal straight to the clock in the brain. Berson et al. 2002 Hattar et al. 2002 Hannibal et al. 2017

Light comes in, the clock resets

Light comes in, the clock resetseyeretinamelanopsin cellsto the clock
Diagram, not to scale. Melanopsin cells are few, scattered across the retina, and talk to the clock through dedicated nerve fibres.

At the wheel

Evening light pushes the clock later. In one study, people who read on a tablet for hours before bed fell asleep later and were less alert the next morning. A study in Basel found that an LED screen, richer in blue light, lowers evening melatonin more than a conventional screen. Chang et al. 2015 Cajochen et al. 2011

02The brain

The conductor

The light signal reaches a small group of neurons at the base of the brain, just above the point where the optic nerves cross. This group is called the suprachiasmatic nucleus, and it is the master clock. Mohawk et al. 2012

From there it keeps time for the rest of the body: for temperature, hormones, hunger and alertness. In the evening it starts the production of melatonin, the hormone of the biological night. During the day it pushes to keep you awake. Mohawk et al. 2012 Arendt & Skene 2005

From the clock to the body

From the clock to the bodytemperaturehormonesalertnesssuprachiasmatic nucleuspineal glandfrom the eyes
Diagram, not to scale. The pineal gland makes melatonin at night, on the clock’s command; light holds it back.

At the wheel

The clock does not know you are driving. At three in the morning it says night, even in the car, even with the radio on. Mohawk et al. 2012

03The two forces

Two forces, one sleepiness

The sleepiness you feel comes from two forces. The first is sleep pressure: it builds with every hour you are awake and only drains away with sleep. One of the molecules that signal it is adenosine: in animal studies, it builds up in the brain during wakefulness and falls during sleep. Porkka-Heiskanen et al. 1997 Landolt 2008

The second is the clock, which acts as a threshold: by day it raises the threshold and keeps you awake even under heavy pressure; at night it lowers it, whatever you have done. The model that brings the two forces together originated in Zurich, with Alexander Borbély, and for more than forty years it has helped explain how sleep works. Daan et al. 1984 Borbély et al. 2016

You can see it clearly in the laboratory: eight men lived for over a month without knowing what time it was, on 28-hour days. Sleep and wakefulness followed both forces at once. Dijk & Czeisler 1995

After a short night you wake up with more pressure, and the margin narrows from first thing in the morning. On a motorway, after a night of only two hours’ sleep, the young drivers in the study crossed the lane markings far more often, right from the start of the drive. Philip et al. 2005

Sleep pressure against the clock’s threshold

Sleep pressure against the clock’s thresholdsleepclock thresholdsleep pressuremarginpast the thresholdreal pressuremore pressure ↑071013161922010407
Two-process model, teaching version: shape, not measurement. Pressure calculated with the time constants of Daan, Beersma and Borbély (1984). Up at 07:00, in bed at 23:00.

At the wheel

In the dead of night the two forces push the same way: pressure is high and the clock is falling towards its minimum, which it reaches before dawn. Coffee helps for a while because it stops adenosine from acting, not because it removes it. Landolt 2008 Philip et al. 2006

04The cell

A clock in almost every cell

Inside almost every cell in the body there is a small clock made of genes and proteins. Two proteins, CLOCK and BMAL1, switch on the PER and CRY genes. The PER and CRY proteins build up, enter the nucleus and switch CLOCK and BMAL1 off. Then they are broken down, the block lifts and the cycle starts again. Takahashi 2017

One cycle takes about a day. The principle of this loop was discovered in the fruit fly; in mammals some parts differ, but the loop works in a very similar way. Takahashi 2017

In Geneva, Ueli Schibler’s group showed that, after a trigger, the clock also runs in rat cells grown in the laboratory, far from the brain. The brain is not the only part of the body that knows what time it is. Balsalobre et al. 1998

The gene cycle

The gene cyclePER and CRY genesCBnucleusone cycle ≈ 24 h

C = CLOCK · B = BMAL1 · ◆ = PER and CRY proteins · lines = gene messages

  1. 1CLOCK and BMAL1 switch the genes on
  2. 2PER and CRY build up
  3. 3they enter the nucleus and switch CLOCK and BMAL1 off
  4. 4they are broken down: the cycle restarts
Diagram of the cycle, not to scale. One cycle takes about a day. The animation shows it in 24 seconds.

At the wheel

Night-time sleepiness is not a thought you can push away. It comes from sleep pressure and from a cycle of clock genes in the brain’s master clock, which keeps running even while you drive. Takahashi 2017 Borbély et al. 2016

05Chemistry

The chemistry of the night

The clock can be read in three curves. Melatonin rises in the evening, as the light fades, and stays high through the biological night. Body temperature falls during the night and reaches its minimum before dawn. Cortisol climbs again towards morning and prepares you to wake up. Arendt & Skene 2005 Hofstra & de Weerd 2008

These are the signals researchers use to find out what time a person’s clock is showing. The shapes shown are typical; the values vary a great deal from person to person. Hofstra & de Weerd 2008 Dijk & Czeisler 1995

Three curves, one clock

Three curves, one clockmelatoninbody temperaturecortisol071013161922010407
Typical shapes, no values: the amplitude varies a great deal from person to person.

At the wheel

Before dawn, low temperature and high melatonin mark the dead of night. In a driving simulator, the hours around two and six in the morning are among the worst of the day for driving. Dijk & Czeisler 1995 Lenné et al. 1997

06The body

The body’s clocks

The liver, kidneys, heart, pancreas and fat tissue have clocks of their own too. They follow the brain, but they also respond to meals. In mice, eating only during the day, the hours when they normally sleep, shifts the liver clock by up to twelve hours, while the brain clock does not move. Damiola et al. 2000 Wehrens et al. 2017

In humans, delaying meals by five hours shifted the blood sugar rhythm by almost six hours and the fat tissue clock by about an hour. The brain clock and sleepiness did not move. Wehrens et al. 2017

Gut bacteria also change over the course of the day, following mealtimes, and jet lag disrupts them. This has been seen mainly in mice. When work and school run against the clock, as in social jet lag, the body is caught between two timetables. Thaiss et al. 2014 Wittmann et al. 2006

Meals delayed by five hours

brainmelatonin and cortisol
fat tissuePER2 gene
blood sugardaily rhythm
Wehrens et al. 2017: ten young men, meals delayed by five hours for six days. Hands that stay still = no significant shift.

At the wheel

The afternoon dip comes mainly from the clock, not from lunch. But in a driving simulator, after a short night, a large lunch made driving worse. Reyner et al. 2012 Lenné et al. 1997

07The road

On the road

Crashes in which the driver fell asleep are not spread at random across the day. In England, police data show three peaks: around two in the morning, around six in the morning and around four in the afternoon. Horne & Reyner 1995

In a driving simulator, driving gets worse at two and at six in the morning, and in the early afternoon there is a dip of similar size. Lenné et al. 1997

After a night shift the risk is real. In a test on a closed track, people coming off a night shift drove much worse than after a night’s sleep, and many drives were stopped for safety. Lee et al. 2016

And at night the road is not empty. On the A2 at three in the morning, an average of 185 vehicles an hour pass through the Gotthard tunnel, 229 at Biasca and 315 at Lugano (FEDRO data, 2022-2025 average).

Three peaks, and a road that never empties

Three peaks, and a road that never emptiespeaks in sleep-related crashes02:0006:0016:00071013161922010407vehicles per hour at 03:00Gotthard185Biasca229Lugano315
Peaks: Horne & Reyner 1995, police data from England. Vehicles: FEDRO, hourly average at 03:00, 2022-2025.

See the full data →

At the wheel

The clock cannot be fooled: not by the open window, not by the radio. You can know where it stands, and stop. On real roads at night, a strong coffee (200 mg of caffeine) or a thirty-minute nap let most young drivers drive almost as well as in the daytime. After sleeping, wait at least a quarter of an hour before setting off: on waking, especially at night, you are less alert for a while. Philip et al. 2006 Reyner & Horne 1998 Trotti 2017

What you can do

Know where the clock stands.Before a trip, check what time you will be at the wheel.Open the tool →Ask yourself one question.“Am I sleepy?” If the answer is yes, believe it.Tired or sleepy? →Stop.An open window and music will get you, at most, to the next rest area: you feel less sleepy, but you barely drive any better. Stop there: a coffee or a short nap.The open window doesn’t wake you →

The journey in seven sentences

  1. Morning light sets the clock right, through cells in the retina that are not for seeing.
  2. A small group of neurons at the base of the brain keeps time for the whole body.
  3. Sleepiness comes mainly from two forces: sleep pressure, which builds while you are awake, and the clock, which acts as a threshold.
  4. In almost every cell, one cycle of genes and proteins takes about a day.
  5. Melatonin, temperature and cortisol follow the clock and tell the body whether it is day or night.
  6. Organs have clocks of their own, which also respond to mealtimes and can drift out of step with the brain’s.
  7. At the wheel, alertness drops most before dawn and, to a lesser extent, in the early afternoon. And the clock cannot be fooled.

The words

Circadian
From the Latin circa diem, “about a day”: an internal rhythm of about 24 hours that continues even without light or timetables.
Suprachiasmatic nucleus
The master clock, at the base of the brain.
Melanopsin
The pigment in some retinal cells: it measures light for the clock.
Adenosine
One of the molecules that signal sleep pressure. In animals it builds up in certain areas of the brain during prolonged wakefulness.
Sleep pressure
The need for sleep that grows with every hour awake. In the model it is called Process S.
Melatonin
The hormone of the biological night. The pineal gland makes it at night, on the clock’s command; light holds it back.
Peripheral clocks
The clocks of the liver, gut, pancreas and other organs.
Social jet lag
The gap between the clock’s timetable and the one set by work and school.

What we know, and what we do not know yet

We know

  • Alertness follows a rhythm of about 24 hours, with its lowest point before dawn and a dip in the afternoon.
  • Sleepiness depends both on how long you have been awake and on the time of day.
  • Evening light pushes the clock later.

We do not know yet

  • How far each person differs from the average curves: chronotype, age and work matter a great deal.
  • How much mealtimes affect sleepiness at the wheel. In a laboratory study of ten young men, delaying meals by five hours did not shift the rhythm of reported sleepiness (Wehrens et al., 2017).
  • Many findings about the body’s clocks come from mice, rats and cats. In humans they are only partly confirmed.

Sources

About the model

The curves at the third stop come from the two-process model. The time constants of sleep pressure are those published by Daan, Beersma and Borbély. The shape and amplitude of the threshold, the limits between wide margin, narrow margin and past the threshold, and the effects of coffee and the nap are our own choices, made to show the mechanism: they do not come from a validated model. The nap, for example, does not show the grogginess of the first minutes after waking. The curves show the typical pattern for someone who gets up at 07:00 and goes to bed at 23:00. They do not measure your sleepiness and do not replace the question “Am I sleepy?” You can change your wake-up time, how much you slept last night and your chronotype: the curves are recalculated with the same constants. Depending on chronotype, the clock shifts about an hour earlier or later, as the body temperature minimum does in morning and evening people (Baehr et al., 2000). The three curves at the “Chemistry” stop also shift with chronotype, in the same way.

01 · Light

  1. Berson DM, Dunn FA, Takao M. Phototransduction by retinal ganglion cells that set the circadian clock. Science 2002;295(5557):1070-1073. DOI 10.1126/science.1067262 · study in rats
  2. Hattar S, Liao HW, Takao M, Berson DM, Yau KW. Melanopsin-containing retinal ganglion cells: architecture, projections, and intrinsic photosensitivity. Science 2002;295(5557):1065-1070. DOI 10.1126/science.1069609 · study in rats and mice
  3. Czeisler CA, Duffy JF, Shanahan TL, et al. Stability, precision, and near-24-hour period of the human circadian pacemaker. Science 1999;284(5423):2177-2181. DOI 10.1126/science.284.5423.2177 · study in people, in the laboratory
  4. Chang AM, Aeschbach D, Duffy JF, Czeisler CA. Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness. PNAS 2015;112(4):1232-1237. DOI 10.1073/pnas.1418490112 · study in people
  5. Cajochen C, Frey S, Anders D, et al. Evening exposure to a light-emitting diodes (LED)-backlit computer screen affects circadian physiology and cognitive performance. Journal of Applied Physiology 2011;110(5):1432-1438. DOI 10.1152/japplphysiol.00165.2011 · study in people, Basel
  6. Hannibal J, Christiansen AT, Heegaard S, et al. Melanopsin expressing human retinal ganglion cells: subtypes, distribution, and intraretinal connectivity. Journal of Comparative Neurology 2017;525(8):1934-1961. DOI 10.1002/cne.24181 · human retina

02 · The brain

  1. Mohawk JA, Green CB, Takahashi JS. Central and peripheral circadian clocks in mammals. Annual Review of Neuroscience 2012;35:445-462. DOI 10.1146/annurev-neuro-060909-153128 · review

03 · The two forces

  1. Borbély AA, Daan S, Wirz-Justice A, Deboer T. The two-process model of sleep regulation: a reappraisal. Journal of Sleep Research 2016;25(2):131-143. DOI 10.1111/jsr.12371 · review, Zurich and Basel
  2. Daan S, Beersma DG, Borbély AA. Timing of human sleep: recovery process gated by a circadian pacemaker. American Journal of Physiology 1984;246(2 Pt 2):R161-R183. DOI 10.1152/ajpregu.1984.246.2.R161 · mathematical model
  3. Dijk DJ, Czeisler CA. Contribution of the circadian pacemaker and the sleep homeostat to sleep propensity, sleep structure, electroencephalographic slow waves, and sleep spindle activity in humans. Journal of Neuroscience 1995;15(5 Pt 1):3526-3538. DOI 10.1523/JNEUROSCI.15-05-03526.1995 · study in people, in the laboratory
  4. Porkka-Heiskanen T, Strecker RE, Thakkar M, et al. Adenosine: a mediator of the sleep-inducing effects of prolonged wakefulness. Science 1997;276(5316):1265-1268. DOI 10.1126/science.276.5316.1265 · study in cats
  5. Landolt HP. Sleep homeostasis: a role for adenosine in humans? Biochemical Pharmacology 2008;75(11):2070-2079. DOI 10.1016/j.bcp.2008.02.024 · review, Zurich
  6. Philip P, Sagaspe P, Moore N, et al. Fatigue, sleep restriction and driving performance. Accident Analysis & Prevention 2005;37(3):473-478. DOI 10.1016/j.aap.2004.07.007 · road, after a short night
  7. Baehr EK, Revelle W, Eastman CI. Individual differences in the phase and amplitude of the human circadian temperature rhythm: with an emphasis on morningness-eveningness. Journal of Sleep Research 2000;9(2):117-127. DOI 10.1046/j.1365-2869.2000.00196.x · study in people

04 · The cell

  1. Takahashi JS. Transcriptional architecture of the mammalian circadian clock. Nature Reviews Genetics 2017;18(3):164-179. DOI 10.1038/nrg.2016.150 · review
  2. Balsalobre A, Damiola F, Schibler U. A serum shock induces circadian gene expression in mammalian tissue culture cells. Cell 1998;93(6):929-937. DOI 10.1016/s0092-8674(00)81199-x · rat cells, Geneva

05 · Chemistry

  1. Arendt J, Skene DJ. Melatonin as a chronobiotic. Sleep Medicine Reviews 2005;9(1):25-39. DOI 10.1016/j.smrv.2004.05.002 · review
  2. Hofstra WA, de Weerd AW. How to assess circadian rhythm in humans: a review of literature. Epilepsy & Behavior 2008;13(3):438-444. DOI 10.1016/j.yebeh.2008.06.002 · review

06 · The body

  1. Damiola F, Le Minh N, Preitner N, Kornmann B, Fleury-Olela F, Schibler U. Restricted feeding uncouples circadian oscillators in peripheral tissues from the central pacemaker in the suprachiasmatic nucleus. Genes & Development 2000;14(23):2950-2961. DOI 10.1101/gad.183500 · study in mice, Geneva
  2. Wehrens SMT, Christou S, Isherwood C, et al. Meal timing regulates the human circadian system. Current Biology 2017;27(12):1768-1775. DOI 10.1016/j.cub.2017.04.059 · study in people
  3. Thaiss CA, Zeevi D, Levy M, et al. Transkingdom control of microbiota diurnal oscillations promotes metabolic homeostasis. Cell 2014;159(3):514-529. DOI 10.1016/j.cell.2014.09.048 · study in mice and people
  4. Reyner LA, Wells SJ, Mortlock V, Horne JA. 'Post-lunch' sleepiness during prolonged, monotonous driving - effects of meal size. Physiology & Behavior 2012;105(4):1088-1091. DOI 10.1016/j.physbeh.2011.11.025 · driving simulator
  5. Wittmann M, Dinich J, Merrow M, Roenneberg T. Social jetlag: misalignment of biological and social time. Chronobiology International 2006;23(1-2):497-509. DOI 10.1080/07420520500545979 · questionnaires

07 · The road

  1. Horne JA, Reyner LA. Sleep related vehicle accidents. BMJ 1995;310(6979):565-567. DOI 10.1136/bmj.310.6979.565 · police data
  2. Lenné MG, Triggs TJ, Redman JR. Time of day variations in driving performance. Accident Analysis & Prevention 1997;29(4):431-437. DOI 10.1016/s0001-4575(97)00022-5 · driving simulator
  3. Philip P, Taillard J, Moore N, et al. The effects of coffee and napping on nighttime highway driving: a randomized trial. Annals of Internal Medicine 2006;144(11):785-791. DOI 10.7326/0003-4819-144-11-200606060-00004 · road, at night
  4. Lee ML, Howard ME, Horrey WJ, et al. High risk of near-crash driving events following night-shift work. PNAS 2016;113(1):176-181. DOI 10.1073/pnas.1510383112 · test track, after a night shift
  5. Reyner LA, Horne JA. Evaluation of "in-car" countermeasures to sleepiness: cold air and radio. Sleep 1998;21(1):46-50. DOI 10.1093/sleep/21.1.46 · driving simulator
  6. Trotti LM. Waking up is the hardest thing I do all day: sleep inertia and sleep drunkenness. Sleep Medicine Reviews 2017;35:76-84. DOI 10.1016/j.smrv.2016.08.005 · review
  7. FEDRO, A2 hourly counts, 2022-2025 · driveawake.ch

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