# Tired but can’t sleep: why it happens

> Feeling tired and being able to fall asleep are governed by two different systems. Sleep pressure builds the longer you are awake; the body clock decides when sleep comes easily, and in the hours before your usual bedtime it pushes hardest for wakefulness. A clock running late, caffeine, bright evening light, a late nap or a mind that stays switched on can each leave you exhausted and wide awake at once.

Source: https://dreamsheeps.space/blog/tired-but-cant-sleep
Published: 2026-10-14 · Updated: 2026-10-14

## Why am I tired but can’t sleep?

Because being tired and being able to fall asleep are run by different systems, and they do not always agree. Sleep science has described the arrangement since 1982 with Alexander Borbély’s two-process model: a homeostatic process — sleep pressure, which builds the longer you are awake and drains away while you sleep — interacting with a circadian process, the body clock, which keeps its own time whether you have slept or not (Borbély, 2022, looking back on more than four decades of the model).

Sleep comes easily when high pressure meets a clock that has turned towards night. When the two are out of step you can be thoroughly worn out — the pressure is real — while the clock is still running the day. [Two processes decide when you are sleepy](https://dreamsheeps.space/learn/two-processes) sets the model out in a few short paragraphs.

It also helps to pull apart two things the word *tired* runs together: feeling drained, and being sleepy — ready to fall asleep if you lie down. The first can be high while the second is low, and that gap is what people mean by *tired but wired*.

## Why do I get a second wind in the evening?

Because the body clock pushes hardest for wakefulness in the evening, not long before your usual bedtime — just as sleep pressure is nearing its peak. In three experiments published in 1986, the Israeli sleep researcher Peretz Lavie had 22 volunteers, most of them after a night without sleep, spend 24 to 36 hours on a 20-minute cycle: 7 minutes trying to sleep, or trying not to, then 13 minutes awake. The pattern was much the same each time. Sleep came most readily at night and, less strongly, in mid-afternoon; between the two lay what Lavie called a “forbidden zone” for sleep, centred on about 8 to 10 p.m. in his volunteers (Lavie, 1986).

A year later, a study of 15 people living in temporal isolation, whose sleep had drifted free of their body-temperature rhythm, found two zones in the circadian cycle in which they rarely fell asleep — and predicted that in everyday life the evening one falls about 1 to 3 hours before habitual bedtime (Strogatz, Kronauer and Czeisler, 1987).

Researchers call that evening stretch the wake maintenance zone: a window of about 3 hours, shortly before the body starts releasing the hormone melatonin. In a 2013 study of 31 young adults kept awake for about 50 hours, performance on tests of attention and speed was better in those 3 hours than earlier in the day, despite the longer time awake — most of all on the second evening, after a night without sleep (Shekleton and colleagues, 2013).

So a second wind is not a sign that you were not tired after all. It is the clock doing what it does in the evening, and it is one reason an early night after a bad one can backfire: it can put you in bed during the very hours Lavie’s sleep-deprived volunteers found hardest to sleep in.

## What if your body clock is running late?

Then the second wind moves with it. The wake maintenance zone is set by the body clock, not by the time on your watch, so a clock running late carries it later too — into the hour you usually go to bed. You arrive tired, because sleep pressure has been building all day, and find the clock still holding the door.

Light is what sets the clock, and evening light pushes it later. In a 2015 study, people who read an e-book on a light-emitting device in the hours before bed, compared with when they read a printed book, took longer to fall asleep, felt less sleepy in the evening, released less melatonin and had a later body clock — and were less alert the next morning (Chang and colleagues, 2015). What counts is the light rather than the device: [light is the clock](https://dreamsheeps.space/learn/light-is-the-clock) explains why morning daylight does more than anything done about screens, and why the useful evening advice is to dim the room, not only the phone.

Irregular hours do the same thing more quietly. A wake time that moves by hours at the weekend moves the clock with it, and Sunday night’s bedtime then arrives before the clock is ready for it.

## What else keeps a tired body awake?

### Caffeine

Caffeine does not clear sleep pressure; it blocks the signal, so the tiredness stays underneath while you feel alert on top. And it lingers. In a 2013 study at home, 400 mg of caffeine disrupted sleep compared with a placebo whether it was taken at bedtime or 3 or 6 hours before, and the authors backed the advice to stop substantial caffeine at least 6 hours before bed (Drake and colleagues, 2013). [Caffeine has a very long tail](https://dreamsheeps.space/learn/caffeine-tail) does the arithmetic.

### A late nap

A nap spends sleep pressure you would otherwise have taken to bed, and the clock will not lend you any back. If you nap, the earlier the better; a doze on the sofa in the evening is the most expensive kind.

## What if it’s your mind that won’t switch off?

Then the problem is not too little tiredness but too much arousal. The hyperarousal model of insomnia holds that in people with long-standing insomnia the body and brain stay switched on, and a 2010 review found raised arousal in them by night and by day, in measures of the nervous system, hormones, the immune system, brain waves and brain imaging (Riemann and colleagues, 2010).

One finding shows what that looks like. In 1988, researchers compared 70 people seeking help for chronic insomnia with 45 people with no sleep complaints: the insomnia group slept less at night, and yet the next day, given chances to nap, they took longer to fall asleep — by the test’s measure, they were more alert (Stepanski and colleagues, 1988). Short of sleep and yet more alert is tired but wired, measured.

Most nights it is less dramatic than that: the day’s unfinished business arriving the moment the lights go out, then the effort of trying to make sleep happen — one of the few efforts that backfire. [Why trying to sleep keeps you awake](https://dreamsheeps.space/learn/trying-to-sleep) explains that loop, and [racing thoughts at night](https://dreamsheeps.space/racing-thoughts-at-night) covers what helps a busy mind, before bed and once you are in it.

## What helps when you’re tired but wired?

Mostly unglamorous things, each aimed at the system that is out of step.

- *Don’t go to bed hours early after a bad night.* It can put you there in the middle of the second wind — the hours Lavie’s sleep-deprived volunteers found hardest.
- *Get daylight early, and dim the evening.* That is how a clock running late is brought back ([light is the clock](https://dreamsheeps.space/learn/light-is-the-clock)).
- *Stop caffeine at least 6 hours before bed*, the cut-off the 2013 study supports.
- *Skip the evening nap*, so the pressure you have built is still there at bedtime.
- *Wait for sleepiness, not just tiredness.* Drooping eyelids and losing the thread of a page are sleepiness; drained and fed up is not. If you lie down and find yourself wide awake, the plan in [what to do when you can’t sleep](https://dreamsheeps.space/blog/what-to-do-when-you-cant-sleep) applies.
- *Give a busy mind something harmless to do* instead of something to stop. [Cognitive shuffling](https://dreamsheeps.space/cognitive-shuffling) is one way: a slow run of ordinary, unconnected images that lead nowhere.

None of this is a treatment for insomnia. For chronic insomnia, the American Academy of Sleep Medicine suggests that clinicians not rely on sleep-hygiene advice like this as a therapy on its own (Edinger and colleagues, 2021); what it recommends is below. [How to fall asleep](https://dreamsheeps.space/how-to-fall-asleep) is the longer guide to ordinary nights.

## When is it more than a bad evening?

When it happens most nights, has gone on for months and is affecting your days. Then take it to a doctor: for chronic insomnia, cognitive behavioural therapy for insomnia (CBT-I) is the recommended first treatment, by the American College of Physicians (Qaseem and colleagues, 2016) and the American Academy of Sleep Medicine (Edinger and colleagues, 2021).

If the trouble is less falling asleep than falling asleep *early* — you can only ever drop off very late, whatever you do — mention that too. A body clock that runs persistently late is a recognised condition, delayed sleep–wake phase disorder, and the American Academy of Sleep Medicine has a clinical guideline for it (Auger and colleagues, 2015).

And if what you feel is not tired but sleepy — dozing off in the day whenever you sit still — that is a different question, and also one for a doctor. [How long should it take to fall asleep?](https://dreamsheeps.space/blog/how-long-to-fall-asleep) explains why dropping off instantly is not the good sign it sounds like.

## Sources

1. [Borbély (2022) — The two-process model of sleep regulation: beginnings and outlook. Journal of Sleep Research 31(4), e13598](https://doi.org/10.1111/jsr.13598)
2. [Lavie (1986) — Ultrashort sleep-waking schedule. III. ‘Gates’ and ‘forbidden zones’ for sleep. Electroencephalography and Clinical Neurophysiology 63(5), 414–425](https://doi.org/10.1016/0013-4694(86)90123-9)
3. [Strogatz, Kronauer & Czeisler (1987) — Circadian pacemaker interferes with sleep onset at specific times each day: role in insomnia. American Journal of Physiology 253(1 Pt 2), R172–R178](https://doi.org/10.1152/ajpregu.1987.253.1.r172)
4. [Shekleton et al. (2013) — Improved neurobehavioral performance during the wake maintenance zone. Journal of Clinical Sleep Medicine 9(4), 353–362](https://doi.org/10.5664/jcsm.2588)
5. [Chang, Aeschbach, Duffy & Czeisler (2015) — Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness. Proceedings of the National Academy of Sciences 112(4), 1232–1237](https://doi.org/10.1073/pnas.1418490112)
6. [Drake, Roehrs, Shambroom & Roth (2013) — Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. Journal of Clinical Sleep Medicine 9(11), 1195–1200](https://doi.org/10.5664/jcsm.3170)
7. [Riemann et al. (2010) — The hyperarousal model of insomnia: a review of the concept and its evidence. Sleep Medicine Reviews 14(1), 19–31](https://doi.org/10.1016/j.smrv.2009.04.002)
8. [Stepanski et al. (1988) — Daytime alertness in patients with chronic insomnia compared with asymptomatic control subjects. Sleep 11(1), 54–60](https://doi.org/10.1093/sleep/11.1.54)
9. [Edinger et al. (2021) — Behavioral and psychological treatments for chronic insomnia disorder in adults: an American Academy of Sleep Medicine clinical practice guideline. Journal of Clinical Sleep Medicine 17(2), 255–262](https://doi.org/10.5664/jcsm.8986)
10. [Qaseem et al. (2016) — Management of chronic insomnia disorder in adults: a clinical practice guideline from the American College of Physicians. Annals of Internal Medicine](https://doi.org/10.7326/M15-2175)
11. [Auger et al. (2015) — Clinical practice guideline for the treatment of intrinsic circadian rhythm sleep-wake disorders: advanced sleep-wake phase disorder (ASWPD), delayed sleep-wake phase disorder (DSWPD), non-24-hour sleep-wake rhythm disorder (N24SWD), and irregular sleep-wake rhythm disorder (ISWRD). An update for 2015. Journal of Clinical Sleep Medicine 11(10), 1199–1236](https://doi.org/10.5664/jcsm.5100)

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Sheeps won’t move your body clock or your coffee. What it does is give a mind that’s still switched on something harmless to hold: a new plotless story, written for you, with no music under the voice, ending in silence. Sheeps on the App Store: https://apps.apple.com/app/id6749510901