Repeated nighttime awakenings can feel like a sign that something is wrong. However, researchers say this can also be a surprisingly common result of what happens in the body before dawn. This has sparked a broader discussion about whether our ancestors considered nighttime wakefulness to be the norm.
It is not about one specific minute. Sleep is not a uniform state throughout the night, and the second half is structurally more fragile than the first. According to a review of sleep physiology on the National Institutes of Health’s NCBI Bookshelf, deep slow-wave sleep dominates the early part of the night, while the proportion of rapid eye movement (REM) sleep in each cycle increases later on; brief awakenings tend to cluster around the transitions into and out of REM sleep.
In young, healthy people, these awakenings are usually so minor that they go unnoticed. As this protective mechanism weakens with age or stress, that same routine transition is more likely to fully wake a person up. Thus, the 3 a.m. awakening may seem strangely specific, even though the mechanism is linked to the structure of the sleep cycle rather than a fixed hour.
Changes in two circadian rhythms also occur during the night. Body temperature reaches its daily minimum—or nadir—in the early morning. According to a review of circadian rhythms by the Centers for Disease Control and Prevention (CDC), this typically happens between approximately 4 a.m. and 6 a.m. Around this same time, levels of cortisol—the hormone most closely linked to wakefulness—begin to rise sharply, approaching the steep spike known to researchers as the “cortisol awakening response,” a phenomenon detailed in expert guidelines published in the journal Psychoneuroendocrinology in 2016. None of these rhythms is tied precisely to 3:00 a.m.; the exact timing depends on an individual’s chronotype and sleep schedule. However, for people following a standard routine, these overlapping shifts often occur in the pre-dawn hours, making sleep shallower just when it is already most vulnerable.
There is a truly interesting, albeit controversial, historical backstory here. Roger Ekirch, a historian at Virginia Tech, documented more than 500 pre-industrial references to “first sleep” and “second sleep”—periods separated by an hour or more of wakefulness—a pattern that largely vanished following the advent of artificial lighting. In the early 1990s, Thomas Wehr, a researcher at the National Institute of Mental Health, tested whether this had a biological basis: sixteen participants—mostly men in their twenties and thirties—spent 14 hours in darkness every night for a month, simulating a natural winter night. By the fourth week, as reported by Scientific American, the participants had adapted to two roughly four-hour sleep periods separated by one to three hours of quiet wakefulness.
This finding did not go unchallenged. A re-evaluation published in the journal Medical History in 2023 argued that the historical and scientific evidence regarding segmented sleep as the dominant pre-industrial pattern is more selective than is commonly portrayed. The article cites a 2013 study in which campers were exposed to natural light and campfire light for two weeks, yet the biphasic sleep pattern was not replicated.
A 2010 study published in The Journal of Psychosomatic Research found that 31.2% of adults already report waking up at least three nights a week. The report suggests that sleep disturbances are simply a common occurrence rather than evidence of a loss of ancestral rhythms.
In short: waking up in the middle of the night has a plausible—though unproven—biological precedent, but it is not yet an established scientific fact.
Sleep specialists consistently offer practical, if unappealing, advice: do not look at the clock or your phone screen, as exposure to light suppresses melatonin production and makes it harder to fall back asleep. Keep the room dark and avoid bright light when using the bathroom. If this is accompanied by daytime fatigue, loud snoring, gasping, or intrusive thoughts, you should consult a doctor; these symptoms could indicate sleep apnea, anxiety, or another treatable condition, rather than a disruption of normal sleep function.
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