Sleep pressure is the drive to sleep that builds the longer you are awake and drains while you sleep. In the two-process model, proposed by Alexander Borbély in 1982, it works with the body clock’s daily rhythm to time sleep. In one study, 17 to 19 hours awake left some test results as impaired as at 0.05% blood alcohol.

Key takeaways

  • The two-process model explains sleep timing with Process S, sleep pressure that rises while you are awake and falls while you sleep, and Process C, the body clock’s daily rhythm.
  • Adenosine is one signal of sleep pressure: in a 1997 Science study in cats, it rose during prolonged waking and fell slowly during recovery sleep. Caffeine blocks its receptors.
  • In a 2000 study of 39 transport and army workers, 17 to 19 hours without sleep left some test results equal to or worse than at 0.05% blood alcohol.
  • The body clock pushes back against sleep pressure through the day, so falling asleep can be hardest in the hours just before your usual bedtime.
  • One change to make: keep naps short and early, and hold a steady bedtime.

What is sleep pressure?

Sleep pressure is the homeostatic drive to sleep: it rises with every hour you are awake and falls as you sleep. Researchers track it through slow-wave activity, the deep, slow brain waves of non-REM sleep, which decline across each sleep episode as the pressure drains.

Chemical signals carry part of it. In a 1997 Science study, Harvard researchers found that adenosine in cats’ basal forebrain, a brain region involved in wakefulness, rose during waking, climbed further across prolonged waking and declined slowly during recovery sleep. A 2022 review in the Journal of Sleep Research calls adenosine a widely accepted natural sleep regulator, and notes that caffeine reduces sleepiness by blocking its receptors.

Long stretches awake have a cost. In a 2000 study in Occupational and Environmental Medicine, 39 transport and army workers were tested across 28 hours without sleep and, separately, after measured amounts of alcohol. After 17 to 19 hours awake, around 10:30 PM to 1 AM, performance on some tests was equal to or worse than at a 0.05% blood alcohol level.

What is the two-process model of sleep?

The two-process model says that when you sleep, and how deeply, depends on sleep pressure (Process S) acting together with a circadian process (Process C) controlled by the body clock. Alexander Borbély of the University of Zurich published it in 1982, and a 2016 reappraisal by Borbély and three colleagues concluded that it remains useful, adding that recordings from the suprachiasmatic nuclei suggest the two processes interact continuously.

Process S: sleep pressure Process C: the body clock
What drives it Time awake The circadian pacemaker in the brain’s suprachiasmatic nuclei
How it moves Rises while you are awake, falls while you sleep A daily cycle that keeps running whether you sleep or not
How researchers see it Slow-wave activity in the sleep EEG Daily rhythms such as core body temperature
What it does Builds the need for sleep Sets when sleep comes easily and when it does not

How do sleep pressure and the body clock work together?

They are timed so you can stay awake all day and asleep all night. The body clock’s push toward sleep is weakest late in the day, when sleep pressure is high, and strongest toward the end of the night, when sleep pressure has drained.

Scientists separated the two with forced desynchrony. In studies by Derk-Jan Dijk and Charles Czeisler at Harvard Medical School, published in 1994 and 1995, eight men lived for 33 to 36 days without time cues on a 28-hour day, so their sleep fell at every point of the body clock’s cycle. Across 175 recorded sleep episodes, the body clock and sleep pressure contributed about equally to keeping sleep consolidated, and the authors concluded that their normal timing is uniquely suited to one solid block of sleep at night and one of wakefulness by day.

Why do you get a second wind before bed?

Because the body clock’s push for wakefulness is strongest in the evening. In a 1987 analysis of 15 people living without time cues, Steven Strogatz, Richard Kronauer and Charles Czeisler found two zones in the body clock’s cycle when people rarely fell asleep. They predicted that in everyday life sleep tendency peaks in the mid-afternoon, and that the hardest time to fall asleep comes about 1 to 3 hours before habitual bedtime.

Peretz Lavie’s 1986 experiments point the same way. After a night without sleep, volunteers spent 24 to 36 hours alternating 13 minutes awake with 7 minutes of trying to sleep or to stay awake. Even sleep-deprived, they found sleep hardest to reach around 8 to 10 PM, which Lavie called a forbidden zone for sleep.

So going to bed hours earlier than usual, even after a bad night, can land you in that window and leave you lying awake. To move bedtime earlier, shift it in small steps, as in How to Fix Your Sleep Schedule: Anchor, Light, 15 Min.

How do naps affect sleep pressure?

A nap drains some sleep pressure, which is why a late nap can make it harder to fall asleep at night. In a 1996 University of Zurich study, healthy young men who napped in the early evening took longer to fall asleep that night and built up less slow-wave activity than on a night without a nap, and a sleep-pressure model closely simulated the change in slow-wave activity.

Short, early naps leave most of the pressure for bedtime. The US National Heart, Lung, and Blood Institute (NHLBI) advises adults to nap no more than 20 minutes and, if they have trouble falling asleep at night, to limit naps or take them earlier in the afternoon. For nap length, see How Long Should a Nap Be? What a 2026 Study Found.

What is the one change to make?

Protect your sleep pressure for bedtime: keep any nap short and early, and go to bed at a steady time instead of hours early after a poor night. This is an everyday habit, not a treatment; ongoing trouble sleeping deserves a conversation with a clinician.

  1. Keep a steady bedtime and wake time, as the NHLBI and the CDC both advise; the CDC recommends 7 or more hours of sleep for adults aged 18 to 60. Weekend drift counts too: see What Is Social Jetlag? What a 2026 Wearable Study Found.
  2. If nights are hard, nap for 20 minutes or less, early in the afternoon, or skip the nap.
  3. After a bad night, hold your usual bedtime rather than going to bed hours early.
  4. Keep caffeine early: the NHLBI notes its effects can last up to 8 hours. See Caffeine Cutoff Time: When to Have Your Last Coffee.

Your day on My BodyClock

What Is Sleep Pressure? The Two-Process Model of Sleep. My BodyClock for Sunday, November 8 in Encinitas: Flow at 6:30 AM (Wake and get ready); Move at 7 AM (Walk outside); Eat at 7:30 AM (Breakfast with coffee); Flow at 8 AM (Commute); Work at 8:30 AM (Focused work); Eat at 12:30 PM (Lunch); Sleep at 1:15 PM (20-minute nap, alarm set); Work at 1:35 PM (Afternoon work); Flow at 5 PM (Commute home); Love at 5:30 PM (Time with family); Eat at 6:30 PM (Home-cooked dinner); Play at 7:15 PM (Hobby, no dozing on the sofa). Center Photo: A calm bedroom with a neatly made bed and sunlight streaming through a large window.
Your day on My BodyClock. Build it in Owaves.

The day above on My BodyClock lets sleep pressure build steadily and spends it at night. A 6:30 AM wake-up and a walk outside start the day, and coffee stays with breakfast. A 20-minute nap after lunch, alarm set, stays within the NHLBI’s 20-minute limit and leaves most of the pressure for bedtime. The evening has no dozing, and lights go out at the same 10:30 PM every night for 8 hours of sleep.

Build your own version in Owaves: My BodyClock on the App Store, or join the Android waitlist. Curious whether your body clock runs early or late? Take the Body Clock Type quiz.

FAQ

What is sleep pressure?

Sleep pressure is the body’s drive to sleep, which builds the longer you stay awake and falls as you sleep. Researchers call it Process S. Adenosine is one of its chemical signals.

What is the two-process model of sleep?

A model, proposed by Alexander Borbély in 1982, in which sleep timing comes from two interacting processes: sleep pressure (Process S) and the body clock’s daily rhythm (Process C).

Why do I get a second wind at night?

Because the body clock pushes hardest for wakefulness in the evening. Research in people living without time cues predicts that falling asleep is hardest about 1 to 3 hours before habitual bedtime.

Do naps reduce sleep pressure?

Yes. In a 1996 lab study, an early-evening nap made it take longer to fall asleep that night. The NHLBI advises naps of 20 minutes or less, taken earlier in the afternoon if you have trouble sleeping at night.

Sources

  1. Borbély AA. A two process model of sleep regulation. Human Neurobiology, 1982;1(3):195-204. https://pubmed.ncbi.nlm.nih.gov/7185792/
  2. 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):131-43. doi:10.1111/jsr.12371. https://pubmed.ncbi.nlm.nih.gov/26762182/
  3. Porkka-Heiskanen T, et al. Adenosine: a mediator of the sleep-inducing effects of prolonged wakefulness. Science, 1997;276(5316):1265-8. doi:10.1126/science.276.5316.1265. https://pubmed.ncbi.nlm.nih.gov/9157887/
  4. Reichert CF, Deboer T, Landolt HP. Adenosine, caffeine, and sleep-wake regulation: state of the science and perspectives. Journal of Sleep Research, 2022;31(4):e13597. doi:10.1111/jsr.13597. https://pubmed.ncbi.nlm.nih.gov/35575450/
  5. Williamson AM, Feyer AM. Moderate sleep deprivation produces impairments in cognitive and motor performance equivalent to legally prescribed levels of alcohol intoxication. Occupational and Environmental Medicine, 2000;57(10):649-55. doi:10.1136/oem.57.10.649. https://pubmed.ncbi.nlm.nih.gov/10984335/
  6. Dijk DJ, Czeisler CA. Paradoxical timing of the circadian rhythm of sleep propensity serves to consolidate sleep and wakefulness in humans. Neuroscience Letters, 1994;166(1):63-8. doi:10.1016/0304-3940(94)90841-9. https://pubmed.ncbi.nlm.nih.gov/8190360/
  7. 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):3526-38. doi:10.1523/JNEUROSCI.15-05-03526.1995. https://pubmed.ncbi.nlm.nih.gov/7751928/
  8. Strogatz SH, Kronauer RE, Czeisler CA. Circadian pacemaker interferes with sleep onset at specific times each day: role in insomnia. American Journal of Physiology, 1987;253(1 Pt 2):R172-8. doi:10.1152/ajpregu.1987.253.1.R172. https://pubmed.ncbi.nlm.nih.gov/3605382/
  9. Lavie P. Ultrashort sleep-waking schedule. III. Gates and forbidden zones for sleep. Electroencephalography and Clinical Neurophysiology, 1986;63(5):414-25. doi:10.1016/0013-4694(86)90123-9. https://pubmed.ncbi.nlm.nih.gov/2420557/
  10. Werth E, Dijk DJ, Achermann P, Borbély AA. Dynamics of the sleep EEG after an early evening nap: experimental data and simulations. American Journal of Physiology, 1996;271(3 Pt 2):R501-10. doi:10.1152/ajpregu.1996.271.3.R501. https://pubmed.ncbi.nlm.nih.gov/8853369/
  11. National Heart, Lung, and Blood Institute. Healthy Sleep Habits. https://www.nhlbi.nih.gov/health/sleep-deprivation/healthy-sleep-habits
  12. Centers for Disease Control and Prevention. About Sleep. https://www.cdc.gov/sleep/about/index.html

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