The more you try, the less it happens?
No, not that, the other one!
You know that feeling when you’ve been awake for 18 hours and your whole body is begging for sleep? That heavy, foggy, full-body pull toward the pillow?
That feeling has a name I like: sleep pressure.
The term traces back to psychiatrist and researcher W.P. Koella in the 1960s. In scientific language it’s called the homeostatic sleep drive. In plain English, it’s the growing need to get some bloody sleep.
This drive increases the longer you stay awake, following what researchers describe as a saturating exponential curve, and it declines exponentially once you’re asleep.
What’s driving this under the hood is a chemical called adenosine. As you stay awake, adenosine gradually builds up in the brain. When enough of it accumulates, it binds to receptors that make you feel drowsy, heavy-eyed, and mentally slower. When you sleep, adenosine clears away, and the pressure resets.
Because of this, sleep pressure has been a topic of interest in sleep science since the early 1900s, especially for people trying to either fall asleep on demand or stay productive across long waking hours.
This mechanism is fundamental to how sleep works. Yet many people who struggle with sleep are unaware of it. Instead, sleep is often framed as something that should happen if you relax enough or apply the right technique.
I’m writing this at 05:40, on the way to a training day in Swindon, aware that sleep pressure as we describe it here was different to the pressure I put on myself to sleep and wake up at a specific time – if I don’t fall asleep now, I’ll be exhausted tomorrow, but I can’t miss the alarm – but that urgency, accompanied by heightened monitoring of the clock and concern about oversleeping, likely reduced sleep pressure rather than facilitated it.
How adenosine builds pressure
Extracellular adenosine concentration fluctuates rhythmically in many brain regions, increasing during wakefulness and decreasing during sleep. Neural activity during the day stimulates the release of adenosine, and prolonged wakefulness leads to increased sleep pressure and subsequent rebound sleep.
Think of sleep pressure like hunger.
If you ate a big meal two hours ago, you’re not going to be ravenous. You might want to eat again, but your body doesn’t need it yet.
Sleep works the same way.
If you napped for two hours in the afternoon, your adenosine levels dropped. By bedtime, you might not have built enough pressure to fall asleep easily, even though it’s “time for bed.”
This is why shift workers, people with inconsistent sleep schedules, and chronic nappers often struggle with insomnia. They’re not building consistent, strong sleep pressure because they’re fragmenting their sleep across the day.
Caffeine: the pressure mask
Caffeine doesn’t give you energy.
Instead it blocks adenosine receptors, temporarily masking sleep pressure without actually reducing it. Caffeine also affects the circadian timing system directly and independently of sleep physiology.
When you drink coffee at 3pm, your adenosine is still building up. You just can’t feel it. Then, hours later when the caffeine wears off, all that accumulated adenosine hits you at once. This is why people crash hard after caffeine – the pressure was there all along, just hidden.
After about 52 hours without sleep, brain imaging studies have shown that adenosine A1 receptors ramp up across the brain, making you feel increasingly wiped out. After a long recovery sleep (around 14 hours), those levels drop back toward normal (Elmenhorst et al., 2007). Your brain is constantly keeping score of how long you’ve been awake and adjusting accordingly. Caffeine can mask that signal, but it doesn’t cancel the debt.
Sleep restriction therapy
Here’s where it gets…counterintuitive.
One of the most effective treatments for chronic insomnia is called Sleep Restriction Therapy (SRT). This procedure is designed to eliminate prolonged middle-of-the-night awakenings by initially restricting the time spent in bed, then gradually increasing it.
If you’re lying in bed for nine hours but only sleeping five, SRT says: only allow yourself five and a half hours in bed. Sounds brutal, right?
But this isn’t just theory. Multiple randomised controlled trials and meta-analyses have shown that sleep restriction therapy works.
Studies consistently find that compared to control groups, people fall asleep faster, spend less time awake during the night, and dramatically improve sleep efficiency and insomnia severity (for example, Miller et al., 2014; Kyle et al., 2015; a large meta-analysis by Weeß et al., 2021).
Even when sleep restriction is used on its own, without the rest of CBT-I (Cognitive Behavioural Therapy for Insomnia, see reading below), the effects hold up. Less time awake in bed builds more sleep pressure, and better sleep follows.
Why “just relax” doesn’t work
When someone with insomnia is told to “just relax,” their “problem” is assumed to be mental. And sometimes it is – anxiety, racing thoughts, hyperarousal all interfere with sleep. But if there’s no sleep pressure built up, relaxation won’t help. You can be perfectly calm and still not fall asleep if your brain hasn’t accumulated enough adenosine to signal that it’s time to shut down.
This is why people can feel “tired but wired”.
The mental fatigue is real – they’ve been awake all day, they’re stressed, they’re burnt out. But if they’ve been napping, drinking coffee in the mid to late afternoon, or spending 10 hours in bed hoping to “catch up” on sleep, they haven’t built the biological pressure needed to actually fall asleep.
Practical takeaways
If you’re struggling to sleep, ask yourself:
Am I actually building sleep pressure?
Are you napping during the day?
Are you going to bed at wildly different times?
Are you spending hours lying in bed awake, which reduces the time you’re actually building pressure during wakefulness?
Are you seeing natural light everyday?
Are you reducing exposure to bright lights before bed?
Are you exercising regularly, having great interactions with people and/or simply getting your steps in?
Is caffeine masking my pressure?
How late are you drinking coffee, tea, or energy drinks?
Are you using caffeine to push through tiredness instead of listening to your body’s signals?
Am I spending too much time in bed?
If you’re in bed for 9 hours but only sleeping 6, you’re diluting your sleep efficiency. Sleep restriction says to match your time in bed to your actual sleep time, then gradually extend it as your sleep improves.
The counterintuitive advice: if you can’t sleep, don’t stay in bed trying.
Get up. Build more pressure. Go back when you’re actually drowsy, not just because it’s “bedtime.”
Sleep restriction builds the need to sleep and works not by forcing it, but by creating the biological conditions where sleep can actually happen.
The bigger picture
Sleep pressure isn’t the whole story.
Circadian rhythms matter. Light exposure matters. Stress, mental health, sleep apnea, restless legs — all of these can disrupt sleep in ways that pressure alone can’t fix.
But for many people lying awake at night wondering why sleep won’t come, the explanation isn’t mysterious or personal failure. It’s often mechanical.
If you’re not sleeping, it may simply be that your brain hasn’t built enough pressure yet.
If you enjoyed this, should we go deeper on the podcast?
How does a conversation with a sleep expert on pressure, circadian rhythms, and what actually works for insomnia sound? If you’ve got questions about any of the above or follow ups on the reading below, send them through!
And that’s it from me!
Sleep well and I’ll see yas in the next one.
Fitness & Thinking in 2026
Forget six packs – we look at six pillars of fitness: physical, mental, social, spiritual, emotional and financial.
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References & Further reading
Core sleep science
Peng, W., et al. (2020). “Regulation of sleep homeostasis mediator adenosine by basal forebrain glutamatergic neurons.” Science, 369(6508). https://www.science.org/doi/10.1126/science.abb0556
Elmenhorst, D., et al. (2017). “Recovery sleep after extended wakefulness restores elevated A1 adenosine receptor availability in the human brain.” PNAS, 114(16), 4243-4248.
Blanco-Centurion, C., et al. (2006). “Adenosine and sleep homeostasis in the Basal forebrain.” Journal of Neuroscience, 26(31), 8092-8100.
Adenosine and caffeine
Urry, E., & Landolt, H.P. (2024). “Adenosine, caffeine, and sleep-wake regulation: state of the science and perspectives.” Journal of Sleep Research, e14120.
Burke, T.M., et al. (2021). “Adenosine integrates light and sleep signalling for the regulation of circadian timing in mice.” Nature Communications, 12, 2113.
Sleep restriction therapy
Spielman, A.J., et al. (1987). “Treatment of chronic insomnia by restriction of time in bed.” Sleep, 10(1), 45-56.
Kyle, S.D., et al. (2014). “Sleep restriction therapy for insomnia is associated with reduced objective total sleep time, increased daytime somnolence, and objectively impaired vigilance.” Sleep, 37(2), 229-237.
Miller, C.B., et al. (2018). “How does sleep restriction therapy for insomnia work? A systematic review of mechanistic evidence and the introduction of the Triple-R model.” Sleep Medicine Reviews, 42, 127-138.
CBT-I overview
Muench, A., Perlis, M.L., & Vargas, I. (2022). “Cognitive Behavioral Therapy for Insomnia (CBT-I): A Primer.” Sleep Medicine Clinics, 17(2), 229-241.
Sleep Foundation resources
“Adenosine and Sleep: Understanding Your Sleep Drive” – https://www.sleepfoundation.org/how-sleep-works/adenosine-and-sleep
“Cognitive Behavioral Therapy for Insomnia (CBT-I): An Overview” – https://www.sleepfoundation.org/insomnia/treatment/cognitive-behavioral-therapy-insomnia