Human evolution News

Why do humans wake up with a start?

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An interesting listicle features ten topics you’d think we’d know the answer to, but don’t. Here’s one:

Here’s #8:

Often when we are about to fall asleep, we experience a kind of a falling sensation which causes us to wake up with a start. It happens to almost everybody, and the sensation is known as a hypnic jerk. It also sometimes happens when you tilt the chair you’re sitting on too far—somehow you can sense when you’re about to fall, and you wake up with a hypnic jerk. We really have no idea what causes them or whether they serve any modern purpose, but science has come up with some interesting theories.One hypothesis suggests that our bodies developed this mechanism when we used to sleep on branches or high ground, and it was meant to help us avoid a fall. But there is no evidence to support it, and humans rarely slept on trees or precarious cliffs as a matter of habit. Other theories suggest that it happens because of the slowing down of the body’s processes when you fall asleep, but again, there are no scientific studies to support that claim either.

Maybe I (O’Leary for News) am reading too much into an interesting listicle.

It’s interesting that this commentary exposes the cultural Darwinblather to critical thinking instead of just accepting it at THE explanation.

It’s indeed unlikely that we or any immediate ancestors, whatever they were like, were in the habit of sleeping on places they could fall off. Humans, unlike cats for example, don’t have any special mechanisms for reducing the harm from falling. And our body size and weight make breaking a limb a serious risk. How much more serious it must have been before modern medicine!

See also: What we know about human evolution

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Amazing: Cats are less likely to be hurt falling from a great height than a small one. Find out why:

6 Replies to “Why do humans wake up with a start?

  1. 1
    Robert Byers says:

    Did science come up with this idea of some people calling themselves science?
    Rather then saying we have this because to avoid falling off branches when we lived on them WHY NOT just say we have in the memory the fear of falling to our hurt??
    its not a jerk but the right response for quick correction. Its in the memory to save ourselves and in the memory falling hurts and in the memory how to quickly fix it. Simple memory math. No different then stopping a puck.

    Falling asleep requires a jerk awake because sleepiness makes one drowsy and not able to deal with a problem. we just make a mistake their is a problem.
    The memory is the sourcem i think, for all strange things.

  2. 2
    polistra says:

    It’s more likely that the jerk comes first, from an unsynchronized relaxation process. When the muscle stretch sensors damp down first, the loosening of muscles doesn’t cause any undue response. When the stretch sensors remain turned on, the loosening of muscles is misinterpreted as a need for adjustment. The adjustment happens suddenly because the secondary feedback is already suppressed. (This sort of thing happens constantly in some forms of CP, where the feedback mechanisms are always mistimed.)

    After the jerk is sensed, the conscious ‘dream-builder’, which always runs about 2 seconds behind the reflexes, has time to build a little ministory about falling and insert the story in front of the consciously sensed jerk.

  3. 3
    Mung says:

    I don’t know about News, but wake up with my cat poking at my nose.

  4. 4
    anthropic says:

    My wife claims she wakes up with a jerk every morning, but really I’m just a little grumpy until my coffee.

  5. 5
    Mung says:


  6. 6
    Robert Byers says:

    I think the simple answer should come first. My answer.
    Your answer is very involved and would need evidence for such a process.
    Remember another example was sitting in the chair.
    I don’t know if any of your stuff is possible but maybe I’m wrong.

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