Long before we had smartphones and electric lights, hardly anyone slept through the night.
It wasn’t a big deal. Across many documented cultures, people went to bed a few hours after sunset, woke somewhere around midnight, stayed up for an hour or so – chatting, tending the fire, visiting neighbors – then drifted into a second sleep until dawn.
Historians call it ‘segmented sleep’. Sleep historian Roger Ekirch has traced more than 2,000 references to it in documents going back as far as Homer’s Odyssey – court records, medical texts, letters, poems.
Writers barely bothered explaining what a ‘first sleep’ and ‘second sleep’ were, because everybody already knew.
Then, around the time of the Industrial Revolution when artificial light pushed bedtimes later, those two sleeps fused into one, and ‘sleeping through the night’ quietly became the standard.
For those of us who regularly wake up at 3 am and struggle to get back to sleep, you’ll know how much it can feel like a failure to succeed at a basic human function.
But what if this period of wakefulness isn’t a malfunction – it’s part of an ancient human sleep habit?
Modern sleep science suggests we were never really meant to sleep an entire uninterrupted night.
Sleep runs in cycles of roughly 90 to 110 minutes, moving through light sleep, deep sleep, and rapid eye movement (REM) sleep. Most of us go through around four to six of these cycles each night.
Deep sleep is front-loaded into the first half of the night, so by the early hours you’re mostly cycling through its lighter stages – the ones easiest to surface from.
Waking briefly is normal; most people do it several times a night and simply don’t remember. It only becomes a problem when the awakening drags on, or when it starts arriving at the same time, night after night.
A 1992 study that had people sleep in a lab without clocks or evening light showed that under those circumstances people often end up reverting to sleeping in two phases.
And a 2017 study of 21 people in a Madagascan agricultural community that doesn’t have access to electric lights overnight found people still mostly slept in two segments.
That precise timing is where it gets more interesting.
Cortisol – the hormone that helps switch your brain into alertness – has long been linked to the jolt of wide-awake clarity that hits at 3 am, as part of its natural rise ahead of morning.
But a 2025 study measured free cortisol in over 200 people using real-time microdialysis in subcutaneous tissue, rather than relying only on saliva samples taken after waking.
It found that the rate at which cortisol rose didn’t actually change at the moment of waking. It kept climbing at much the same pace whether someone was asleep or awake.
In other words, the “cortisol awakening response” everyone cites might not be a response to waking at all.
It could just be your circadian rhythm doing what it always does, waking or not, while your cortisol rises steadily anyway. Still, keep in mind, the researchers themselves note the variability between people is huge, and a lot of this is still unsettled.
For some people, the overnight wake-up really does have a more specific cause.
A 2026 review in the journal Physiologia looked at sleep disruption during the menopause transition, which affects an estimated 40 to 60 percent of women going through it.
As estrogen and progesterone decline, the drop in progesterone alone can undercut deep sleep, while overnight hot flushes and a resurgence of cortisol in the second half of the night combine to make sleep hard.
Blood sugar can do something similar. If dinner was early or light, glucose can dip overnight. The body responds by releasing stress hormones to push it back up – a mechanism sometimes strong enough to wake you outright, most often in people with diabetes but not exclusively.
Caffeine, even if it was last consumed in the afternoon, can increase the chance of waking. A 2025 review showed that just two standard alcoholic drinks were enough to disrupt REM, with the impact getting more intense the more you consume.
So while your 3 am waking habit may not be that abnormal, where it does become a genuine problem is the loop that can build around it.
Lying awake worrying about being awake is a real, measurable type of rumination. And after enough nights in a row, the brain starts associating 3 am with alertness rather than rest – the mechanism behind some chronic insomnia. Checking the clock feeds it. So does lying there getting frustrated.
So what does work?
Having good sleep hygiene is important – avoiding blue light, alcohol and caffeine ahead of falling asleep, and having a predictable wind-down routine can all help you sleep more soundly.
Research has also shown that a consistent waking time anchors the body clock better than an earlier bedtime does.
Related: There Are 5 Profiles of Sleep – Here’s What Yours Says About Your Health
If you’re still awake after twenty minutes or so, getting up and doing something dull until you feel sleepy again can break the bed-equals-wakefulness link.
Cognitive behavioral therapy for insomnia targets the same loop directly, and remains one of the most effective tools going.
In the meantime, rest assured that if you’re awake and worrying at 3 am, you’re certainly not alone, and there are a lot of biological reasons behind your wakefulness.
This article was fact-checked by Rebecca Dyer and edited by Rebecca Dyer. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.
